The aim of this multiple case-study analysis was to find out the influence that service-learning methodology has on the development of sustainability competences amongst university students (Preschool, Primary and Social Education) of three Spanish Universities (UIC, UAM and USAL). A sample of 129 university students of the academic year 2017-18 was used. A pre-experimental study was carried out, using a pre-test-post-test study with natural groups, without a control group. The findings of this study show that service-learning contributed to improve knowledge in sustainability in the 5 case studies analysed. The findings also demonstrate how the use of service-learning promotes the development of practical skills associated with sustainability action.