This chapter presents a more detailed description of challenger quality and its measurement. It describes the dynamics of challenger quality between 1972 and 2018, and demonstrates the decline in challenger quality between 1972 and 2018. Despite a rich literature on candidate quality and—more broadly—on candidacy and candidate emergence, few analyses track developments over time in contemporary elections. The chapter considers developments in US Senate races and open seat races for the US House to illustrate some points. Researchers interested in politicians' career paths are often guided by ambition theory, initially developed and applied to electoral politics by Schlesinger. Strategic politicians will consider both national and district-level factors during the decision-making process and are more likely than inexperienced candidates to wait for a seat to become vacant before tossing their hat into the electoral ring. The chapter also presents an overview of the key concepts discussed in this book.