Up to 80% of factors that determine quality, costs, environmental impact and market success of a product are determined during the conceptual design phase. Environmental friendliness of a product under development is one of the many important evaluation criteria to consider during concept selection. Once the concept of a product is selected for further development, improvements to the environmental profile of the product are limited to the embodiment, detailing, and life cycle design changes. Environmental impact assessment is the most widely accepted approach to evaluating environmental friendliness of a product. However, methods such as Life Cycle Assessment (LCA) are not suitable for environmental evaluation in the conceptual design phase and comparison of original design concepts. LCA provides quantitative analysis of environmental impacts of products and processes, but it requires detailed and complete product information which is generally not available in the conceptual phase. This thesis proposes environmental criteria and eco-evaluation method for technical systems in conceptual phase. Motivation, research questions, research aims and methodology are introduced in Chapter 1 of the thesis. Development of environmentally friendly products and solutions is a major driver for innovation and product development. This is particularly important in cases of new and sustainable alternatives replacing well-established, but environmentally costly products. Two main approaches to eco-evaluation in the conceptual design phase; qualitative and quantitative eco-evaluation are described in Chapter 2 of the thesis. Qualitative eco-evaluation is concept evaluation based on evaluation of product's attributes according to environmental criteria. Quantitative eco-evaluation is based on life cycle analysis, environmental impact approximation or LCA of similar technical systems used as a reference. Ecodesign methods and tools prescribe using a combination of qualitative multi-criteria eco-evaluation approach and quantitative analytical life cycle assessment. Usually, numerous attributes of the final solution and its life cycle need to be assumed a forehand to perform quantitative eco-evaluation of conceptual solutions. Prejudiced interpretation of environmental friendliness of concepts may lead to negative outcomes such as selection of environmentally unfavourable concept, which later may cause additional costs to the development process. Environmental impact assessment in the conceptual design phase can be managed without performing LCA by exploiting the commonalities between different product systems. However, currently available environmental impact approximation methods do not support comparison of original design concepts which are generally concepts without predecessors. Thus, it is usually not manageable to find similar comparable reference products. Further, quantitative eco-evaluation in conceptual phase would require embodiment design to be developed and analytical verification of anticipated life cycle performance of the product. Eco-evaluation should be neither costly, nor time-consuming and support evaluation of product concepts that potentially integrate new, innovative and original solutions. A qualitative approach to eco-evaluation is thus more suitable for the conceptual design phase. As qualitative approaches to eco-evaluation are far less studied, generally more prone to the prejudiced interpretation of environmental friendliness by product developers and better suited for conceptual design phase when most information relevant to the future product are qualitative, aims of the research reported in this doctoral thesis are: 1. development of environmental criteria to enable eco-evaluation of technical systems in the conceptual phase of product development, 2. development of eco-evaluation method based on environmental criteria, and 3. enabling eco-evaluation and comparison of technical systems in the conceptual design phase. Case study on eco-evaluation of laundry cleaning concepts using ecodesign guidelines as environmental criteria is described in Chapter 3. Appropriateness of ecodesign guidelines to be used as environmental criteria is investigated in the case study. Mechanical engineers were given a task to evaluate environmental friendliness of five laundry cleaning conceptual solutions and to rank the concepts accordingly. In the first part of the case study, evaluation criteria were not proposed to evaluators, so consequently obtained rankings were based on their subjective preferences and personal notions on environmentally better and worse concepts. In the case of two concepts, a paradoxical situation occurred, known as Arrow's paradox where some evaluators ranked one product concept as one of the least environmentally friendly concepts and other evaluators ranked the same concept as one of the best environmentally friendly concepts. Concept rankings were generated by evaluators individually, so the case study was focused on evaluators working independently. In the second part of the case study, the same set of concepts was eco-evaluated. This time evaluators used ecodesign guidelines as environmental criteria and a datum ranking method. Different rankings of concepts have been established and results of the two sets of rankings are compared. There was more dissimilarity in rankings of less environmentally friendly concepts in the first part of the case study. One of the reasons for this outcome may be a lack of information on environmental criteria that need to correspond to all the various product concepts presented to evaluators. Then, there is a lack of environmentally related information about the likely performance of those concepts. Thirdly, personal preferences and experiences guided evaluators in their choice of more and less eco-friendly concepts. The introduction of eco-evaluation criteria in the second part of the case study has yielded more coherent rankings. Results of the case study show that using ecodesign guidelines as environmental criteria is a better option than not using any criteria during concept evaluation. However, most of environmental criteria prescribed by ecodesign guidelines refer to product attributes which were not available in the description of laundry washing concepts in the study. According to widely available models of the design process, technical systems' concepts can be described by their functions, physical effects, working principles and principle solutions. Embodiment and detailing are usually performed after the concept is selected for further development as the most promising solution. Detailed information about concepts is usually not available in the conceptual design phase. A question arises on how to enable environmental evaluation in the conceptual design phase since information about the product at this phase are mostly qualitative, product key features and embodiments are not finalized and the life cycle of the product is unknown or vaguely defined. Environmental criteria and eco-evaluation method are proposed in Chapter 4 of the thesis. As the concept of a product can be represented and modelled as high-level abstraction model where functions of the product are thought of as transformation entities, each concept is described by chains of energy, material and signal transformations which enable particular physical effects. Energy, material, and signal transformed in the technical process are called operands. Since chains of physical effects and secondary effects towards the environment can be deduced from the technical process, environmental friendliness of a technical system can also be known by assessing the properties of operands transformed in the technical process. The hypothesis of doctoral dissertation is: eco-evaluation of a particular technical system can be managed by assessing properties of operands transformed in the technical process. Specifically developed within the thesis to support product concept eco-evaluation, five environmental criteria is the basis of the method proposed to support technical systems' concept eco-evaluation. Chains of effects and secondary effects towards the environment contain input data for the method proposed. Eco-evaluation is performed by evaluating energy, material and signal transformations indicated by chains of physical effects and secondary effects towards the environment which are outputs from the technical process. Theoretical background of energy transformity effectiveness criterion is emergy theory. According to emergy theorists, quality of different energy forms, materials, and services (environmental, human and economic) can be evaluated on a common basis by conversion of Joules, calories and other energy units to a unified form of available energy. In this case, the chosen energy units are solar emergy units called solar transformities. Solar transformities can be defined for conversions of solar energy units to other energy form units (wind kinetic energy, tide energy, Earth crustal heat), use of resources (coal, fuel, wood) and some other flows which do not have to be exclusively energy, but are a result of previous energy or resource utilization processes such as human services and information. Solar transformity factors are quantifiable in form of solar emergy units per unit of useful energy in corresponding energy form. Solar emergy units can be calculated by including all energy and resource utilization processes to produce useful energy in a desirable energy form. Then, a hierarchy of energy forms can be composed and it includes energy forms of lower and higher quality, ranked in an orderly list format. Inspired by emergy theory, favourable energy transformations are energy transformations where low-quality energy forms are used up to realize the work potential of high-quality energy forms. Energy transformity effectiveness criterion considers the type of energy form transformations that are indicated by the functions of the product (for example, conversion of electrical to mechanical energy, electromagnetic to heat, mechanical to acoustic energy etc.). Energy transformity of a particular energy conversion process is a coefficient of solar transformity of output energy form and solar transformity of input energy form. Energy transformity effectiveness is a sum of all energy transformities of energy transformations indicated by the chains of physical effects inherent to the concept of the technical system. Energy transformity can be defined only for functions of the product indicating that there is a change (conversion of or transformation) in the energy form between input energy flow and output energy flow. For some energy forms, energy transformity factors are very large, so transformity factors are transformed into a logarithmic scale which simplifies calculation. The second environmental criterion is a minimizing criterion of the total number of energy, material and signal transformations. An optimal solution is characterized by the high value in energy transformity effectiveness and the low total number of energy, material and signal transformations. The criterion of eco-quality of energy, material and signal waste or emissions corresponds to evaluating secondary effects towards the environment, e.g. output flows of energy, material, and signal to the environment. This is an eco-criterion for evaluating material operand transformations that are non-intended secondary outputs (e.g. waste, pollutants, and emissions). The criterion is based on adopted concepts of waste management hierarchy, levels of waste toxicity and environmental friendliness of end of life treatments. To each option, a value from a twenty-point scale is assigned. The scale provides sufficiently fine measurements without being overly precise. It is linear and ranges from -10 to 10. Values of zero, 5 and -5 are not specified to give a more rigorous value distribution between more and less prosperous solutions. Eco-quality of energy, material and signal waste or emissions equals sum of eco-qualities of all output flows. Criterion of number of material, energy and signal waste or emissions corresponds to evaluating the number of different output flows of energy, material, or a signal which can be deduced from the function structure or chains of effects of the concept. An optimal solution is characterized by the high value of eco-quality of energy, material and signal waste or emissions and low value of total number of energy, material and signal waste or emissions, e.g. outputs to the environment potentially resulting in environmental impact. Fifth environmental criterion is the number of transformations in which material changes states (solid, gas, liquid). The change of material state indicates that additional energy is required for the transformation to occur. It also points out some special material properties required for principle solutions supporting the transformation (such as heat resistance), and as such are indicative of additional or lower environmental impact. Proposed eco-evaluation method is based on five environmental criteria, rank-sum rule to aggregate the criteria outcomes, and is supported by a decision matrix. Eco-evaluation by using the proposed method produces a ranking of concepts indicating environmental friendliness of concepts in a comparable way. Environmental criteria and eco-evaluation method proposed are specifically developed for qualitative eco-evaluation in the conceptual design phase. Environmental criteria and method proposed are also the main theoretical and practical contributions of the work reported in this doctoral thesis. Methodological contribution is achieved by providing validation of proposed method by means of results from a case study on eco-evaluation using ecodesign guidelines, LCA study of laundry cleaning concepts and Validation Square – a method for validating design methods. Effectiveness and efficiency of environmental criteria and eco-evaluation method proposed are demonstrated on two illustrative set of examples. The first set examples are laundry cleaning concepts. Results LCA of laundry cleaning concepts are described in Chapter 5, and these are compared to eco-rankings produced by mechanical engineers in a case study. The second set of concepts used to test the proposed eco-evaluation method is based upon rankings comparison of three alarm clock concepts: mechanical, electro-mechanical and digital alarm clock. Environmental friendliness of alarm clock concepts has been based on data from a study on redesign of alarm clock concepts available from literature. Examples of using the proposed method confirm that the proposed method supports qualitative approach to evaluating conceptual solutions and provides support for original design concepts. The effectiveness of the proposed method is confirmed by comparing eco-rankings of laundry cleaning concepts produced by engineering designers in a case study, eco-rankings of the same concepts based on LCA results and eco-rankings resulting from applying the proposed method to the same set of concepts. Chapter 6 describes requirements regarding theoretical and empirical suitability and acceptability of the proposed method for conceptual phase of technical system development. Some of the limitations of the proposed method are that it requires additional effort from users regarding concept analysis, identification of chains of physical effects, and secondary effects, and the users are at risk of inconsistent valuation of secondary effects. Conclusions and future work are described in final chapter of the thesis.