Drilling and well completion technology have progressed to make it possible to drain reservoirs with multiple branching wellbores connected into one tubing string - usually known as multi-lateral wells. However, the economic benefit of multi-lateral wells has not been reliably understood up to now because of uncertainties in their cost and performance. This is in part due to technical difficulties in predicting flow performance and drainage efficiency. A number of operating companies in the North Sea have sponsored a research and development project to tackle these difficulties. This paper, based on that project, shows how analytical and reservoir simulation methods have been combined to predict the productivity and sweep efficiency for simple cases in homogeneous reservoirs. The results have then been extended to cover irregular well geometries in realistic heterogeneous reservoirs. The paper concludes with an assessment of the potential and anticipated benefits of applying multi-lateral wells in a range of reservoir types characteristic of the North Sea. The emphasis is on the evaluation processes used.