To remain sustainable, some open source projects need a constant influx of new volunteers, or newcomers. However, the newcomers face different kinds of problems when onboarding to a project. In this paper we present the results of a systematic literature review aiming at identifying the barriers that a newcomer can face when contributing to an Open Source Software project. We identified and analyzed 21 studies that evidence this kind of problem. As a result we provide a hierarchical model that relies on five categories of barriers: finding a way to start, social interactions, code issues, documentation problems and newcomers’ knowledge. The most evidenced barriers are newcomers’ previous technical skills, receiving response from community, centrality of social contacts, and finding the appropriate way to start contributing. This classification provides a baseline for further researches related to newcomers onboarding.