Best practice suggests that a modular enterprise architecture, where interfaces between and among business processes and services are standardized, is a key IT capability for firms to achieve profitable growth. But few firms have successfully designed, implemented, and maintained such an architecture. This article presents findings on the drivers, constraints, and actions taken by two companies that transitioned to a modular enterprise architecture in response to competitive pressures from their business partners or customers. One company implemented an industry standard and the other developed custom partner interface processes (PIPs) to achieve business modularity. The lessons from these two case studies show how companies can smoothly transition to a modular enterprise architecture.