This paper presents a conceptual framework for fostering teacher professionalism in Government primary schools in Karachi, Pakistan. The framework identifies various stages for enhancing and sustaining teacher professionalism. These stages have been derived from in-depth multiple case studies of four Government primary schools in Karachi where reforms have been occurring. In order to enhance and sustain professionalism among the teachers they need to be provided with opportunities to expand their capabilities and, consequently, the reform managers will need to encourage them through various stages of development. Amongst the various stages are those of involvement, initiation and engagement and developing collaborative school cultures. The theory building provided in the paper will be extremely important in the current context of Pakistan where several initiatives are being undertaken to reform and improve the Pakistani Government school system. The sustainability of such reforms requires the building of teachers’ professional capacity and enhancing their professionalism.