This chapter reviews results of recent studies about children and youths' understanding of advertising in general and social marketing communication in Hong Kong and China. It reports how children and youth perceive the purpose of social service marketing, the comprehension of key messages in selected public service advertisements, and the criteria in differentiating social marketing from marketing of a commercial nature. These studies include both qualitative interviews and quantitative surveys. Children and youth's interpretation of two government publicity campaigns on green living and food waste reduction were used as case studies to illustrate factors that contributed to an effective communication. The chapter ends with an elaborated discussion of how public service advertising was created and managed in Hong Kong through the Information Services Department. The chapter provides insights for social services marketing in communication to children and youth in Hong Kong. Social marketing uses the principles and processes of commercial marketing to promote socially beneficial behavior change (Evans, 2006). Because nearly all societies are keen to socialize new members (e.g., their children), children are a major target audience of social marketing. Social marketing usually involves the promotion of abstract ideas, rather than concrete products and services; thus, it is natural to ask if children understand these messages. At what age will they begin to be interested in social marketing communications? How does this interest develop with time? What forms of social marketing communication appeal most to children? This book chapter attempts to answer these questions.