Assignment of staff to work tasks is a complex problem that involves a large number of factors and requires a lot of expertise. Long term as well as short term requirements need to be met which demands flexible solutions. Software tools can aid planners in reaching optimal dispatching plans but currently available solutions provide only incomplete support. This thesis describes the design, development, and evaluation of a prototype for semi-automated assignment planning called A-Plan. We have carried out this work in the context of a gas device maintenance provider. In A-Plan, assignments of service technicians to customers are displayed visually and can be modified by direct manipulation. Smooth cooperative work is possible and an optimization algorithm has been integrated that facilitates semi-automatic planning. A qualitative evaluation with potential users and IT professionals provided encouraging feedback on the proposed integration of automated methods and interactive visual interfaces.