Outdoor microcosms and mesocosms are useful in risk assessment when lower-tier and higher-tier laboratory studies (single-species and multi-species) indicate potential risks. They can be an important tool in bridging the gap between these studies and attempts to understand, predict and confirm what may occur in the natural environment subject to these risks. Every microcosm or mesocosm study should be designed to test a specific hypothesis using information gained in previous steps of the risk assessment. Although this guidance document is mainly based on experiences with testing of plant protection products (PPP), in principle, it applies to other groups of toxic substances as well (e.g., wastewater, industrial chemicals). However it should be noted that suitable exposure design for substances other than PPPs can differ (e.g., steady contamination instead of single application).