This investigation of the implementation of a positive youth development program (Project P.A.T.H.S.) was part of a large study undertaken comprehensively to explore how effective the Tier 1 Program was in practice and how the results can shed light on future developments. Utilizing a case study approach, individual and focus group interviews were conducted in 2007 to examine the factors that influence the process and quality of implementation of the Tier 1 Program of the Project P.A.T.H.S. The focus of this study was on how the implementers of a school made use of the experience gained in the Experimental Implementation Phase (EIP) in 2005/06 to improve the program implementation quality in the Full Implementation Phase (FIP) in 2006/07. Results showed that the program implementation in the FIP was generally high and the program was well received by the implementers. Factors that facilitated the implementation of the program were identified, including the adoption of an incremental change strategy, the incorporation of the program into both formal and informal curricula, positive perceptions of the program among staff and agency social workers, sufficient school administrative support, excellent cooperation between the school and the social work agency, presence of a dedicated school contact person and instructors who engaged themselves in continuous quality improvement of the implementation, and an emphasis on application of what had been learned. Difficulties encountered by the school in the process of implementation were also observed. Based on the present findings, key process variables that facilitate or impede the implementation of positive youth development programs are discussed. Implications for future program implementation are also discussed.