The concept of regionalization of wastewater treatment has been suggested as an effective means of meeting the water quality goals at a minimum cost. Although economy of scale provides the primary incentive for regionalization, other advantages may be gained. Such advantages may include flexibility in operation, more qualified personnel, and high degree of automation. The selection of an optimal regional plan requires a considerable variety and quantity of information, a great expenditure of time and money, and a wide diversity of expertise. The optimal plan provides the least cost system of collection, treatment and disposal of wastewater within a region. In the study, the scope of the regional planning problem is defined. Critical factors influencing the selection of the optimal plan are identified and discussed. Several optimization techniques used in selecting the least cost plan are presented. Data needs are described.