This manual defines methods for estimating selectivity of gillnet and longline gears, with particular reference to the use of these gears in abundance surveys. The manual covers how gear design, the basic concepts of size selection and fish morphometric data might be used to model selectivity. Statistical estimation methods are described in detail, and examples given of their application in spreadsheets. The efficient use of these gears in abundance and research surveys is also discussed. Althou gh the manual concentrates on gillnets and longlines, the methods given are general enough to be relevant to any research involving gear selectivity.