There is a growing interest in the use of combined cycles for power generation. These cycles are known to have economical advantages over classical steam cycles under some circumstances. This thesis looks at the implications of topping a existing steam cycle with one or more gas turbines. After a short overview of combined cycles, the various options available for repowering are presented and discussed. The number of feasible basic options is reduced to two by considering technical issues related to classical steam cycles. A computer code that was developed to analyze the design point as well as the off design performance of repowered configurations is then presented. Design point performance examples show what can be achieved by repowering, and how it compares with genuine combined cycles. Off design performance examples illustrate how the off design behaviour can be improved by special gas turbine features and arrangements. An economic analysis method based on cash flow predictions and sensitivity analysis is developed and illustrated on two typical situations. Throughout the work, the emphasis is put on the necessary integration of technical and economical analysis for decision making.