This chapter examined the implementation quality of the Tier 1 Program (Secondary 3 Curriculum) delivered in the Experimental Implementation Phase of the Project P.A.T.H.S. (Positive Adolescent Training through Holistic Social Programmes). Process evaluation was conducted in the form of systematic observations of curriculum units in four randomly selected schools. Results indicated that the overall level of program adherence was high, ranging from 80% to 100%, with an average of 90%. The mean ratings of the program implementation quality were high, and the inter-rater reliability on these ratings across the observers was highly reliable. Despite limitations, the findings of this study suggest that the implementation of the Secondary 3 Program (Tier 1 Program) of the Experimental Implementation Phase was of very high quality. This provides strong evidence to account for the successful and encouraging outcomes of a major positive youth development program in Hong Kong. Copyright © 2011 by Nova Science Publishers, Inc.