Two types of Organization Development theory are identified and discussed. The first, implementation theory, focuses on the intervention activity required to carry out effective planned change efforts in organizations. The second, change process theory, attempts to explain the dynamics through which the organization changes as a response to any intervention activity. Evaluations of both types of theory suggest that implementation theory is more fully developed than change process theory. However, improved theoretical development is needed in both areas. For implementation theory, further work is needed in the development of more comprehensive diagnostic models, in the specification of conditions for effective change, and in the establishment of useful criteria for change agents. Since change process theory is in such a fragmented state, all facets of its require further theoretical advancement. Substantial improvement in the quality of both types of OD theory are needed to enhance the efficacy of planned change programs in organizations.