Acoustic instrument calibration is fundamental to the quantitative use of its data for estimating aquatic resource abundance. Regular calibrations also allow instrument performance to be monitored to detect changes due to the environment or component dynamics, degradation, or failure. This is the second ICES Cooperative Research Report (CRR) focussed on calibrations of acoustic instruments. The first, CRR No. 144 (Foote et al., 1987), was published during the era of analogue electronics more than a quarter of a century ago. Since then, not only has the acoustic equipment improved vastly with digital electronics and signal processing, but the techniques for applying them to studies of marine organisms have both advanced and diversified. Motivating, facilitating, and expediting these developments is the work of the Fisheries Acoustics, Science and Technology Working Group (WGFAST) of the International Council for the Exploration of the Sea (ICES). CRR No. 144 guided the fisheries acoustics community to uniformly apply the sphere method to calibrate survey equipment, generally single-frequency, split-beam echosounders. Today, surveys of fishery resources are conducted using a large variety of acoustic instruments including, but not limited to, single-frequency, multifrequency, single-beam, split-beam, broad bandwidth, and multibeam echosounders; side-scan and scanning sonars; acoustic Doppler current profilers; and acoustic cameras. These instruments differ in the ways in which they function, are utilized, and the types of measurements they provide. In most cases, they also require different calibration techniques for optimizing the accuracy and characterizing the precision of the measurements. With technological innovation proceeding at an ever faster pace, the challenge to create a comprehensive and practical guide to calibrating acoustic instruments is formidable. Obviously, not all acoustic instrumentation and methods are addressed here. The ones that are addressed are in various states of maturity. Therefore, the practical aims of this CRR are to document (i) acoustic instruments currently used in fisheries research and surveys, (ii) theoretical principles of calibrating these instruments, and (iii) methods currently being practiced for a selection of commonly used instruments. To meet these goals, the WGFAST formed the Study Group on Calibration of Acoustic Instruments (SGCal) at its meeting in April 2009. The SGCal first met in San Diego, CA, USA in April 2010 to outline the document. Some chapters were drafted intersessionally. The SGCal met for the second time in Reykjavik, Iceland in May 2011 to collectively review some draft chapters. The drafts were refined intersessionally and merged. The draft CRR was collectively reviewed at meetings of the SGCal, in Pasaia, Spain in April 2013 and in New Bedford, MA, USA in May 2014. Multiple independent reviewers provided input, and the final editing was completed in 2014. The authors hope that this CRR will be a valuable reference to both novice and experienced users of fishery acoustic instruments, but recognize that it is a provisional guide that requires refinement and update as the field continues to progress.