Adolescent-Friendly Health Services (AFHSs) are those that are accessible, acceptable, equitable, appropriate and effective for different youth sub-populations. This study investigated the process through which four clinics in two countries - Peru and Ecuador - introduced, developed and sustained AFHSs. A multiple case study design was chosen, and data from each clinic were collected through document review, observations and informant interviews. National level data were also collected. Data were analysed following thematic analysis. The findings showed that the process of introducing, developing and sustaining AFHSs was long term, and required a creative team effort and collaboration between donors, public institutions and health providers. The motivation and external support was crucial to initiating and sustaining the implementation of AFHSs. Health facilities' transformation into AFHSs was linked to the broader organisation of country health systems, and the evolution of national adolescent health policies. In Peru, the centralised approach to AFHSs introduction facilitated the dissemination of a comprehensive national model to health facilities, but dependency on national directives made it more difficult to systemise them when ideological and organisational changes occurred. In Ecuador, a less centralised approach to introducing AFHSs made for easier integration of the AFHSs model.