Much recent research in evolutionary developmental biology has focused on the origin of new body plans. However, most evolutionary change at the population and species level consists of tinkering: small-scale alterations in developmental pathways within a single body plan. Such microevolutionary events have been well studied on a population genetic level and from the perspective of adaptive phenotypic evolution, but their developmental mechanisms remain poorly studied. This book explores both theoretical and practical issues of tinkering. It features a wide range of perspectives to address several fundamental questions. How does tinkering occur developmentally, and how is it manifested phenotypically? Are the developmental mechanisms by which tinkering occur different from those that underlie larger evolutionary changes? What are the developmental constraints on tinkering? And how do we test hypotheses about microevolutionary shifts in development from the fossil record? With contributions from experts in a range of fields, this book is for anyone studying evolution, developmental biology or genetics.