In the past, behavior was assumed to be largely invariant within species, particularly those elements of behavior used as criteria of mate choice or in species recognition (see Magurran this volume, Verrell this volume). As is obvious from this volume, geographic variation could well be the common condition rather than the exception, and this applies to the full spectrum of behavioral phenotypes. Not only must students of behavior avoid typological thinking (Mayr 1963), but those wishing to infer similarity of behavior among populations must demonstrate the similarity just as surely as those interested in exploring population differentiation must demonstrate the differences. Behavior is as much a phenotype as is morphology; it is the expression of the combined effects of genotype and environment. Like other traits, behavior varies geographically because it is subject to geographically varying conditions and, hence, to natural selection, gene flow, and genetic drift. The chapters in this book provide examples of this variation, of the underlying genetic bases for the differences, and in many cases, the causes of the geographic variation. The study of geographic variation in behavior is in very early stages and lags well behind research on geographic variation in other kinds of traits (Endler 1977, 1986, 1995). Consequently, we cannot answer with assurance many of the questions we would like to be able to answer. However, we can take a first step using the insights offered by the research presented in this book. Before doing so, we briefly address some of the methodological issues that emerged over the course of the research because many are specific to the study of behavior or of geographic variation. We hope this will help others avoid problems encountered in these early studies. Many of the methodological issues discussed in the chapters in this book are related to the difficulty of working with behavioral characteristics that are extremely labile and responsive to environmental conditions. The remainder are issues related to the interpretation of data collected to assess patterns and causes of geographic variation. We will examine them in turn.