Arguments based on historical practice are a mainstay of debates about the constitutional separation of powers. Surprisingly, however, there has been little sustained academic attention to the proper role of historical practice in this context. The scant existing scholarship is either limited to specific subject areas or focused primarily on judicial doctrine without addressing the use of historical practice in broader conceptual or theoretical terms. To the extent that the issue has been discussed, accounts of how historical practice should inform the separation of powers often require “acquiescence” by the branch of government whose prerogatives the practice implicates. Such acquiescence is commonly seen as critical for historical practice to have the force of law. Yet the concept of acquiescence has been treated much too casually in the literature. Claims about acquiescence are typically premised on a Madisonian conception of interbranch competition, pursuant to which Congress and the executive branch are each assumed to have the tools and the motivation to guard against encroachments on their authority. It has become apparent from political science scholarship, however, that the Madisonian model does not accurately reflect the dynamics of modern congressional-executive relations. This fact necessitates a reexamination of the premises and implications of the idea of institutional acquiescence in particular, and of the role of historical practice more generally. Ultimately, we argue, the problems with the Madisonian model are not fatal to crediting historical practice in interpreting the separation of powers. But they do require more attention to the reasons why such practice is invoked, the extent to which these reasons demand institutional acquiescence, and the precise method by which such acquiescence is identified. To illustrate the importance of each of these questions, we present three case studies of constitutional debates concerning the separation of powers in which practice-based arguments are prominent—war powers, congressional-executive agreements, and removal of executive officers.