According to Nelson Polsby, the US Congress differed from most of the world’s legislatures because of its transformative capacity. The factors that account for the differences in the policy-making power of legislatures is a central question in legislative studies. One such factor is a legislature’s ability to attract and retain talented and ambitious politicians. The US Congress, for example, has presumably maintained its separateness, independence, and influence due to its ability to nurture legislative careerism. This chapter examines legislative careers in democratic legislatures and how they are influenced by political institutions. It first considers early efforts to define and measure legislative careers, along with the relationship between political institutions and legislative careers. It then looks at the evolution of research on legislative careers from macro-level studies of career patterns to micro-level studies of career choices. It also discusses several research frontiers, including research on legislative term limits, and concludes by identifying a number of key unanswered questions as well as the challenges faced by researchers in addressing them.