Saalfeld and Strøm (Chapter 18) show that legislative parties are not only virtually ubiquitous, but also highly institutionalized, typically having well-developed structures of leadership as well as specialization. The authors review various explanations of political parties, including functional theories as well as the explanations based on the incentives of individual legislators. They also discuss challenges to such theories like the influential work of Keith Krehbiel on U.S. Congressional parties. Saalfeld and Strøm argue that it is meaningful to differentiate between various degrees of partyness among parties, and even among coalitions of parties, and review the institutional sources of variations in partyness, including the agenda control available to party leaders and the rewards and sanctions they control.