Without the confidence that patent rights can be enforced quickly and efficiently, when needed, the patent system will not stimulate innovation. For this reason, governments, academics, international institutions and the private sector have poured significant resources into gathering and analysing statistics on patent enforcement over the last two decades. This paper reviews these studies and finds that while infringement is relatively common, much enforcement occurs informally and less than 1–2% of patents incur litigation. New strategic uses of the enforcement system, especially by nonpracticing entities, are the major emerging enforcement issue, especially in the United States. While the old problem of litigation costs attracts the lion's share of empirical attention, it has produced remarkably few solutions to date.