Mechanistic effect models (MEM) have become increasingly popular in the field of environmental risk assessment (ERA) of plant protection products. However there remains numerous open questions to risk assessors, modelers, applicants and the public. This report provides a scientific evaluation of 11 MEM or MEM families. It also critically assesses their relevance for ERA (e.g. as a refinement tool), based on case studies. Finally, the report offers suggestions for future improvements in model development for implementation in ERA. Outcomes were discussed with experts from industries, consultancies, academia and agencies at a European symposium (19-20 Sept 2019, Berlin); the presentations and minutes are available in the annex.