This article provides a systematic review of the empirical literature on barriers within public sector innovation processes, based on data from 63 articles. We investigate the nature of barriers. The studies were analysed based on four dimensions of barriers: i) their classification; ii) their interrelations; iii) whether they play distinct roles within stages of innovation process and iv) whether they vary in the types of innovations. We develop an empirically based framework to capture the complex nature of barriers. For this purpose, a new classification is also introduced to show that interaction-specific barriers emerge during the collaborative innovation process. Significantly we identify that the nature of barriers are more complex than has previously been recognized: they differ in process stages and innovation types. Moreover, they show interrelations across the innovation process by reinforcing each other. The findings show there is an emphasis on organizational barriers and implementation phase studies. We conclude with a discussion on how future research use quantitative and cross-national methods to examine: (1) interaction specific barriers, in particular, barriers with businesses and political bodies (2) design and sustainment phases of innovation (3) interrelations between barriers.