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. 4 the concept of innovation in a normative positive way. Similarly, we do not approach the concept of barriers in a normative negative way. The remainder of this article is structured as follows. The first section details the methodology used for the systematic review. The second section and its subsections detail the results of the systematic review, initially providing details of the studies included and subsequently exploring the nature of barriers by examining their types, process characteristics, interrelations, and distinct variations on types of innovation. Finally, from our findings, we reach conclusions and provide a research agenda on barriers to PSI. METHODOLOGY Database Search Three strategies were used to identify eligible articles. First, we carried out an electronic search in Web of Science restricted to Social Sciences Citation Index (SSCI) in title, abstract and keywords of articles. The term [innov*] was searched along with the terms for barriers: [obstacle* OR barrier* OR challeng* OR hinder* OR deter* OR difficult* OR bottleneck* OR problem* OR setback* OR hurdle* OR hamper* OR imped* OR obstruct*]. To define the public sector character of innovation we use the following terms: [municipal* OR "public administrat*" OR "public organi$ation*" OR "public management" OR government* OR "public service" OR "local government*" OR "e-govern*" OR "egovern* OR governance OR "public sphere" OR "public sector"]. Public or policy entrepreneurship has been studied connected with innovation (Roberts and King 1996) and so we searched the terms for barriers and public sector with the combination of [entrepreneur*]. This strategy resulted in a total of 3,524 articles. Second we selected EBSCO Business Source database ...