With the ever growing use of nanoparticles in a broad range of industrial and consumer applications there is an increasing likelihood that such nanoparticles will enter the aquatic environment and be transported through freshwater systems, eventually reaching marine waters. In addition, large population centres in near-coastal areas increases the probability of nanoparticles rapidly reaching estuarine and marine waters due to direct outflow of wastewater streams. Due to silver’s known antimicrobial properties and widespread use of silver nanoparticles (AgNP) in consumer products, the latter’s environmental fate and impact is therefore of particular concern. In this context, the aim of this doctoral study is to characterize AgNP in sea water and investigate the species-specific effects of low, environmentally-relevant, concentrations and different sizes of AgNP on marine invertebrate: sea urchin embryos of the species Arbacia lixula, Paracentrotus lividus and Sphaerechinus granularis. Further, pyrethroid insecticides, which are widely used today in a diverse range of applications from agriculture, to nurseries, urban structural and landscaping sites, residential homes' garden pest control and mosquito abatement, also have the potential to enter wastewater streams and coastal waters. The potenital interaction between AgNP and these insecticides may significantly increase the toxicity of the latter. Thus, this doctoral work also includes investigation of the effect of AgNP in combination with two commonly used pyrethroid insecticides, cypermetrin and deltametrin, in bivalve Mytilus galloprovincialis as a model by determination of the activity of two enzymes in mussel gills: acetylcholinesterase and glutathione S-transferase, and the concentration of metallothioneins in the digestive gland.