This paper analyses and reports on the action processes of a Hong Kong secondary school adopting a multidisciplinary, project approach. It is written from an observer's perspective exploring the initiation, the nature, the learning activities and the assessment tasks of the student projects, teacher development programmes as well as the technical aspects of implementation. The author has observed how practitioners problematised and reconstructed habitual practices in a cyclical mode where they 'plan-act-observe-reflect' on their daily professional experience and developed practical knowledge. The yearlong processes depict key elements of an action research (Kemmis, 1988; Elliot, 2003). It is found that teachers' tacit knowledge had a significant impact on early identification of problems and suggesting solutions to ensure the smooth running of the curriculum. The learning logs and verbal presentations provided evidences that students had a sense of achievement wherever there were independent learning and decision-making. Both teachers and students reported team spirit, student-teacher rapport, and reflected enhanced self-efficacy. This matches what Katz and Chard (2000) describe as the '(cultivation) of the life of the young child's mind', which means not only knowledge and skills, but also social, emotional, moral, aesthetic and spiritual sensibilities. This paper also illustrates other multifaceted benefits to the school. Conscious of the impact of action research and the contributions of teachers' tacit knowledge in professional practice, the author argues that it is time for university academics or experienced researchers to help teachers publicize the 'tacit' to enrich the knowledge base for teaching and learning.