Middle childhood and early-adolescence—from about 6 to 12 years of age—is a life span during which strong changes on biological, cognitive, affective and social level occur (Collins, 1984). Children develop life competencies and make experiences that affect their health and well-being throughout their adult lives (Eccles, 1999; Huston & Ripke, 2006). Children’s and adolescents’ social and emotional skills, connectedness with peers and adults, school experiences, physical health and well-being are important protective factors and assets for children’s and adolescents’’ general well-being, academic and life achievement and general health (e.g., Hertzman & Power, 2006). This paper presents results from the application of the Middle Years Development Instrument (MDI) in Canada and Switzerland. The MDI was developed, implemented and validated in Canada. It was then translated into Italian and applied in the Italian speaking region of Southern Switzerland (Canton Ticino). Through the MDI, specific subjective well-being areas are investigated, namely, social and emotional development, connectedness, school experiences, physical health and well-being.