This book provides an introduction to coevolution in both microevolutionary (ecological) and macroevolutionary (historical) time. It emphasizes the integration of cophylogenetic, comparative, and experimental approaches for testing coevolutionary hypotheses. Recent work in coevolutionary biology has been successful in demonstrating coadaptation between species in response to reciprocal selection. Fewer studies have tested the influence of coadaptation on the diversification of interacting taxa. We review studies that have attempted to do just this. The overriding question addressed is “how do ecological interactions influence patterns of codiversification?”. We focus on the coevolution of interacting species, particularly those involving external parasites that live on hosts. Such parasites include a diverse assemblage of organisms, ranging from herbivorous insects on plants, to monogenean worms on fish, to feather lice on birds. Ectoparasites are powerful models for studies of coevolution because they are easy to observe, mark, and count. Many of the examples in the book involve parasitic lice of birds and mammals. Lice and their hosts are unusually tractable systems for studies that attempt to integrate coevolutionary ecology and history. Some chapters in the book are very broad in scope, introducing coevolutionary concepts that apply to all interacting species. Other chapters are more narrowly focused on the biology and coevolution of lice and their hosts. The overall goal of the book is to integrate coevolutionary concepts with examples of empirical tests of coevolutionary theory in micro- and macro-evolutionary time. The book concludes with a framework for better integration of coadaptation and codiversification.