The rise of internships as a form of work experience that students pursue during their degree or after graduation has been accompanied by an upsurge of discussions, critical and favourable, on the role of internships for interns' employment opportunities. There is a need, however, to understand the learning that goes in internship as for many students internship is a setting where work practices are encountered for the first time. Recently it has been suggested that unaccredited internship can be seen as constituting a separate work activity that needs to be examined in its own right. The aim of this article is to contribute to this literature by focusing on the learning challenges that arise in unaccredited internship and identifying the capacities that interns develop as a result of tackling these challenges. To that end, I identify a set of analytical concepts from vocational learning literature developed to understand the challenges and opportunities associated with learning across contexts (i.e. education and work): horizontal expertise, boundary-crossing, recontextualisation and identity-renegotiation. Then I analyse data on learning in unaccredited internships collected from five focus groups and two interviews (18 interns). A dialogic discourse analysis of focus group and interview discourses revealed that the interns in unaccredited internship developed an emerging capacity to learn and work competently across multiple contexts and to initiate and coordinate subsequent cycles of boundary-crossing between education and work. The paper proposes the notion of "interns' horizontal expertise" to describe this emerging capacity that arises from learning in unaccredited internship and continues after the internship and explains how this concept differs from other expressions of horizontal expertise in the literature such as the horizontal expertise of seasoned professionals in inter-professional activities and boundary-crossing in work placements.