Engineered nanomaterials are ubiquitous in the environment. They transform and are in  interaction with their environment. As the environmental conditions change, so do their mobility and positive and negative environmental effects . These materials generate a number of concerns regarding their safety. As a result stakeholders, policymakers, NGOs and the general public look for answers how these materials affect our environment. Standardized tests generally do not account for the complexity of a multi-parameter biogeochemical context with its variable/evolving spatial- and temporal conditions. To circumvent these limitations, mesocosm experimentation, i.e. utilizing miniature ecosystems in a lab setting, provides a valuable tool to monitor the fate and effects of natural- and engineered nanomaterials under nature-like conditions. The scope of this session is to address the most recent conceptual and technical advances associated with mesocosm testing and identifying key issues to be resolved with respect to existing testing strategies.