Data are collected from a variety of acoustic systems in many countries to address a range of ecosystem monitoring and stock management objectives. A key step in the analysis of fisheries acoustics data is target classification, i.e. categorizing the backscatter data, ultimately by target species, so that it can be converted into estimates of abundance or biomass. The information needed to classify acoustic targets may be contained within the acoustic measurements, particularly if they are made over a range of frequencies. The SIMFAMI project, financed by the European Union, presented some multifrequency methods for species identification (Fernandes et al., 2006). Readers should also note that there are two other ICES reports on related topics: CRR No. 238 Report on Echo Trace Classification (Reid, 2000) and Acoustic seabed classification of marine physical and biological landscapes (ICES, 2007). However, as these reports were written when multifrequency and wideband methods were less mature, they mostly focus on single-frequency methods. Acoustic classification of biological targets is a fast-moving field. While most of the theoretical principles in the earlier reports are still relevant, there is a need to evaluate recent developments, expand their applications to contemporary technologies, and recommend target-classification protocols for use in fisheries research and ecosystem surveys. Several ICES Member Countries and observer countries have identified these needs and conveyed them to ICES Working Group on Fisheries Acoustics, Science, and Technology (WGFAST) and Science Committee (SCICOM). This is the first ICES CRR to detail the latest multifrequency and wideband methods for acoustic target classification.