This study is a qualitative inquiry into the theory of action of school development planning (SDP) constituted within the professional culture of the School Development Planning Initiative (SDPI). SDP is delineated as an historically contingent term of art most influential in scholarly and policy discourse under the auspices of the Education Reform Act (1988) in the United Kingdom and the Education Act (1998) in Ireland. SDP in Ireland reflects national policy aspiration and traditional Irish cultural and educational values. SDPI, though only established for eleven years, had a crucial role as an agency of the then Department of Education and Science in promoting and supporting SDP in Irish secondary schools in fulfilment of statutory obligations and a national agenda of school improvement. This study presents a qualitative thematic analysis of documentary material and interview data. Using qualified grounded theoretical analytic techniques, the analysis produces findings showing that the primary goal for SDPI was the development of collaborative, deliberative professional cultures among teachers as self-conscious learners, facilitated by supportive leadership, focused upon enhanced pupil learning. The findings also chart an historical pattern of shifting priorities for SDPI in building capacity for SDP until school self evaluation discursively displaced SDP, culminating in a more instrumentalist model of planning. The study also identifies competing loci of control and power between central instrumentalist and accountability expectations on the one hand, and, on the other, the promotion of school autonomy and teacher empowerment. These competing loci of control form the pivotal historical axis of the problematisation of SDP in the study. The findings give weight to the argument that this antinomy is a core determinant of SDPI’s theory of action. The relationship of SDPI to the inspectorate reflects this tension. Conceptualising the inner culture of SDPI the thesis identifies both strong cultural cohesion and creative licence. Key features of valorised SDP derive from the internal culture of SDPI experienced as a community of practitioners. However, strategic naivity and conflicted loyalties to school communities and the Department of Education and Skills contributed to the decline both of SDP as a leading term of art in school improvement discourse in Ireland and to SDPI as a pivotal programme of support for schools. In relation to categories derived from Argyris and Schon, SDPI displays model 2 behaviours operationally, but model 1 behaviours strategically. The thesis contributes to an understanding of an important phase of recent school improvement practice in Ireland, including the relationship of professional culture to praxis, and the need for alignment of purpose among key agencies in school improvement policy design and implementation.