This thesis describes data integration in heterogeneous systems, with an emphasis on applications in enterprise environment. E-business through web services is covered, including B2B integration, as well as B2C integration. Integration of applications allows better data control and improves internal processes. Enterprise Architecture Planning is the first step in achieving high quality information for all organizations. In this phase the correlation between current and new features and functionalities is defined. The goal of EA is to ensure the alignment of information technology to business strategy. It is extremely important that all parties have the same understanding of the new integrated system. The standards, frameworks of business processes and architecture will enable a high-quality overview of all relevant factors. Data quality is a very important prerequisite for decision making at the operational, tactical and strategic business levels. The metadata that describes the data, provides a clear definition and understanding of data, for all parties within the definition phase, construction phase and usage of the data warehouse in the production phase. The data warehouse is one of the elements of information systems, which enables the data integration from external and internal sources. The processes of data transformation, data cleaning and data verification enables us to build high-quality databases with the capability of monitoring data through time. Business intelligence tools are components of decision support systems. The most important are data mining and Online Analytical Processing tools. OLAP tools provide insights and cross-section data across multiple dimensions. Data mining allows the gathering of new knowledge from large amounts of data. Leveraging of integrated data in a data warehouse is described using an example in marketing, showing how integrated data help with the planning and realization of new offers. Integrated data are used for decision making in all phases, from ideation, feasibility, conception and realization. It is also shown that integrated data are extremely important in improving internal business processes by using integrated systems.