The objective of the present document is to provide national control programmes with up-to-date guidelines on monitoring the efficacy of anthelminthic drugs administered in preventive chemotherapy programmes against schistosomiasis and soil-transmitted helminthiases. Guidance is provided on when and how to assess the efficacy of anthelminthics, including detailed recommendations on indicators of efficacy, sample size, follow-up period, laboratory methods, statistical analysis and final interpretation of data collected, and also on how to respond when drug efficacy is reduced. In addition, examples are provided of an information letter for schools, an informed consent form, standard operating procedures for all recommended laboratory methods and a form for data collection. The method described here is for evaluating the efficacy of a single anthelminthic drug against a group of parasites. It is not recommended for drug combinations (e.g. albendazole + praziquantel or albendazole + ivermectin).