Internships are rapidly becoming integral to the school-to-work transition. Yet too little is known as to their effectiveness in providing a bridge to longer-term employment. Previous studies have repeatedly found a strong positive association between interns being paid and their post-programme labour market integration. However, convincing evidence is lacking on the causal mechanisms underlying this association. Data from two international surveys allow us to investigate the source of the differential impact of paid vs. unpaid internships. Our analysis of the first survey data confirms the existence of a robust positive association between the payment of interns and beneficial post-internship outcomes. The second allows us to go one step further and identify some of the specific characteristics underlying `successful' internships. In particular, we find that internships that are structured, last at least six months, involve a mentor and provide health insurance produce better post-internship outcomes than those lacking these features.