Background: Although outdoor smoking bans at school are becoming important, it remains unclear whether successful implementation is feasible and what conditions promote it. Therefore, this study evaluates the implementation process by identifying important factors. Methods: Interviews were held with directors of 24 secondary schools that had decided to implement an outdoor school ground smoking ban, to identify important factors during implementation. Additionally, observations were made before and (about 4 weeks) after implementation to evaluate adherence to guidelines. Data were analyzed using a thematic approach. Results: During implementation, the following items were important--smoking ban (e.g., guidelines), school (e.g., policy, enforcement, environmental factors, resources, etc), and external factors (e.g., social environment). Four schools were unable to implement the ban, 20 schools implemented the ban but student adherence was low, and at 4 schools, staff members still smoked on school grounds. Conclusions: Although implementation is feasible, there are challenges. Schools need to apply a phased/dynamic process with ample lead time and clear communication of the policy. Directors should deal with offenders by strictly enforcing the ban and helping them stop smoking. This study offers information to help institutes implement an outdoor smoking ban to further protect adolescents from the harm of smoking.