Groundwater monitoring is the highest tier in the leaching assessment of plant protection products inthe EU. The European Commission requested EFSA for a review by the PPR Panel of the scientificpaper of Gimsing et al. (2019) on the design and conduct of groundwater monitoring studies. ThePanel concludes that this paper provides many recommendations; however, specific guidance on howto design, conduct and evaluate groundwater monitoring studies for regulatory purposes is missing.The Panel notes that there is no agreed specific protection goal (SPG) at EU level. Also, the SPG hasnot yet been operationalised in an agreed exposure assessment goal (ExAG). The ExAG describeswhich groundwater needs to be protected, where and when. Because the design and interpretation ofmonitoring studies depends on the ExAG, development of harmonised guidance is not yet possible.The development of an agreed ExAG must therefore be given priority. A central question in the designand interpretation of groundwater monitoring studies is that of groundwater vulnerability. Applicantsmust demonstrate that the selected monitoring sites represent realistic worst-case conditions asspecified in the ExAG. Guidance and models are needed to support this step. A prerequisite for theregulatory use of monitoring data is the availability of complete data on the use history of the productscontaining the respective active substances. Applicants must further demonstrate that monitoring wellsare hydrologically connected to thefields where the active substance has been applied. Modelling incombination with (pseudo)tracer experiments would be the preferred option. The Panel concludes thatwell-conducted monitoring studies provide more realistic exposure assessments and can thereforeoverrule results from lower tier studies. Groundwater monitoring studies involve a high workload forboth regulators and applicants. Standardised procedures and monitoring networks could help toreduce this workload.