The aim of this book is to outline the main techniques for ecological censusing. There are many reasons for deciding to carry out a census. It may be to determine the importance of a site, the population size of a species, the habitat requirements of a species, the reasons for the species decline, or whether habitat management has been a success, or to understand the population dynamics. In this chapter I will outline the different approaches necessary for tackling each of these questions and describe some common mistake to collect far more samples that can ever be analysed.