[Bronwyn H. Hall, Christian Helmers, Georg von Graevenitz and Chiara Rosazza Bondibene] Executive Summary: This report analyses whether entry of UK enterprises into patenting in a technology area is affected by patent thickets in the technology area. The aim is to contribute to our understanding of the role of patent thickets as a barrier to entry into new technologies for UK enterprises, in particular small and medium sized enterprises (SMEs). The report consists of several parts: 1) a review of the literature on patent thickets, including the limited empirical evidence regarding effects of patent thickets on R&D investments and competition; 2) discussion of the factors contributing to thicket formation and growth; 3) an empirical evaluation of the extent to which patent thickets appear to be barriers to entry in some technology areas.