Although the concept of modularity is pervasive across fields and disciplines, philosophers and scientists use the term in a variety of different ways. This paper identifies two distinct ways of thinking about modularity, and considers what makes them similar and different. For philosophers of mind and cognitive science, cognitive modularity helps explain the capacities of brains to process sundry and distinct kinds of informational input. For philosophy of biology and evolutionary science, biological modularity helps explain the capacity of random evolutionary (...) processes to give rise to highly complex and sophisticated biological systems. Although these different ways of thinking about modularity are largely distinct, this paper proposes a unifying feature common to both: isolability, or the capacity of subsystems to undergo changes without resulting in substantial changes to neighboring or interconnected subsystems. (shrink)