The Working Group on Biological Effects of Contaminants (WGBEC) investigates the biological effects of contaminants in the marine environment. The group works to increase the understanding of the effects of contaminants in the marine environment and of their links with the presence of chemical pollutants, updating available methods, and generating integrated strategies for monitoring biological effects, which serve to support international research and monitoring. The WGBEC has maintained for decades a list of recommended methods for marine monitoring and, in collaboration with the “Study Group on developing new guidelines for the monitoring of biological EFFects of contaminants” (SGEFF), the WGBEC has recently reviewed the current guidelines for biological effects monitoring, which are urgent requirements for maintaining national marine pollution monitoring programs. In this period the group has also coordinated an ICES Techniques in Marine Environmental Sciences manuscript on “Nuclear abnormalities in mussel haemocytes and fish erythrocytes”, which was published in October 2022. The group also has continuous activity on quality assurance of biological effects methods, which is relevant to ensure that the data collected are of the highest quality. During this period, a BEQUALM interlaboratory comparison exercise for the biomarker EROD (ethoxy‐resorufin‐O‐deethylase), was coordinated by the group. These intercalibration exercises are part of a continuous activity of the group and provide an important function, ensuring national laboratories are following the correct procedures and that the results produced by one laboratory are comparable with others. Also, the group has reviewed the effects of microplastic particles on marine organisms, expanding this activity to other types of natural and synthetic particles present in the marine environment. The group has also carried out collaborative work with the MCWG and the WGML, on chemical additives in plastics, which has led to a scientific publication. Another relevant activity of the group is to update national activities on biological effects‐based monitoring, allowing different approaches to be evaluated and existing programs to be harmonized. The group has conducted an overview of national effect‐based monitoring programmes with the aim to harmonize these programmes aiming to support European countries and Regional Seas Conventions on their implementation. A summary of the national effects‐based monitoring programmes has been provided. In this period, the group has contributed to the OSPAR Quality Status Report 2023, with the report “Integrated biological effects and chemical contaminants approach ‐ a case study”. The group has also contributed to novel scientific issues such as the effects of contaminants of emerging concern, reviewed methods to assess the bioavailability and effects of contaminants on sediment‐dwelling organisms, evaluated species differences in biological effects monitoring, and addressing the effects of mixtures of chemicals on marine organisms.