We examine thirty years of computer industry market structure. Our analysis explains the persistence of dominant computer firms, their recent decline, and the changing success of competitive entry. It emphasizes the importance of technological competition between computer ‘platforms’, not firms. This aspect of competition has changed little over time. Two things did change. Young platforms serving newly founded segments eventually challenged established platforms across segment boundaries through a process of indirect entry. Vertically disintegrated platforms have led to divided technical leadership in important segments. The result is an industry with far more technological competition.