Silver nanoparticles (Ag NPs), tiny materials of dimensions less than 100 nanometers, are increasingly incorporated into consumer products due to their antimicrobial properties. The extensive applications and use together with the not yet fully understood properties of Ag NPs have raised concerns over the environmental and human health. This thesis aimed at the evaluation of potential hazards of Ag NPs and silver ions released from them and at the identification of some key factors that determine toxicity. An integration of different approaches was employed using standard bioassays with aquatic organisms, studying relevant mechanisms of action, followed by the identification of target tissues and organisms of Ag NP exposure and an integration of sensitive techniques to identify subtle changes in responses. Both the ions released and the Ag NPs themselves contribute to toxicity depending on NP size and synthesis method. However, effects were usually observed at concentrations above expected environmental Ag NP levels. Future research should focus on more realistic experimental conditions at concentrations likely to occur in the environment.