The state of mathematics and science education in South Africa is a cause for concern. This situation can be attributed, in part, to many mathematics and science teachers' limited content knowledge, ineffective teaching approaches, and unprofessional attitudes. To address these three problem areas simultaneously, a holistic model for the development of Grades 10 to 12 Physical Science teachers was constructed and evaluated against national and international benchmarks. The effects of the model were assessed over a period of four years with 75 teachers. The model was developed in a distance education context, with no face-to-face contact required. It comprises the following elements: a study guide which integrates the development of teachers' content knowledge, pedagogical content knowledge, cognitive skills and experimental skills; reflective journals; assignments; workshops; peer support and science kits. We briefly describe the research that culminated in the Holistic Professional Development (HPD) model, followed by an account of each element of the model. We then present evidence that suggests that the model is effective in helping teachers develop along three desired dimensions, namely, content knowledge, teaching approaches, and professional attitudes.