Defining terms is a serious undertaking and one bound to stir controversy. When we decided to devote a book to this task, we were under no illusion that everyone would readily sign on to our proposal. We thought, however, that it was worth calling attention to certain conceptual problems that currently beset the study of American political development (APD). We wanted to underscore the value of tractability in claims about change over time and to demonstrate the benefits of specificity in moving this venerable research tradition forward. We regret that Professor Thomas finds our proposal objectionable, but we are even more concerned that his alternative seems to do little to address the issues that prompted our effort.