This paper defines and examines three types of strategic momentum. Repetitive momentum occurs when organizations repeat previous strategic actions. Postitional momentum occurs when organizations take actions that sustain or extend existing strategic positions. Contextual momentum occurs when general traits, such as organizational structure, shape strategic action in a consistent fashion. Event-history analysis of 262 large firms over a 29-year period indicates that: (1) the occurrence of mergers tends to increase the rate of mergers of the same type (repetitive momentum), and (2) organizational decentralization increases the rate of diversifying mergers (contextual momentum). Product market diversification was found to increase the probability of product extension mergers but not conglomerate mergers, only partly confirming positional momentum. The results indicate that internal momentum can affect merger activity, and suggest the importance of continuing research on the role of inertia in organizational adaptation.