Chapter 1 provides an overview of the book. It shows descriptively how the ideology of congressional candidates changed from 1980 to 2012. I focus on the decline in liberal Republicans and conservative Democrats in the supply of congressional candidates during this time. I review leading explanations for partisan polarization in Congress, and I include a brief section on asymmetric polarization. I also discuss how electoral and non-electoral factors shape the calculus of candidacy and the decision to run for office, and I explain why non-electoral factors, specifically party fit and the value of the office, are increasingly relevant in the current partisan context. The quantitative and qualitative data that are used throughout the book are detailed as well, and I conclude with an outline of the chapters.