The purpose of sampling the bottom fauna of freshwater habitats is usually to discover which species are present, and to detect spatial and temporal changes in the density of each species. The purpose of the present guide is to give an elementary account of some suitable statistical methods. Aimed at non-statisticians, this book has been written by a zoologist who found it necessary to teach himself enough about these techniques to apply them with confidence to the particular problems he encountered in analysing his data. These methods provide information on the spatial distribution of the bottom fauna, they enable estimates to be made of the total population in an area of bottom, and they provide a sound basis for judging the significance of quantitative differences between samples. All methods are described in detail with a minimum of mathematical theory, and all data for the examples were collected from small streams in the Lake District in North West England.