Scientific and technological development over the past 20 years has been tremendous. It presents the military planner with a dazzling array of possible developments to meet a wide range of operational requirements. Unfortunately, in every country resources are limited and a choice between the various alternatives has to be made. An objective and, if possible, quantitative method of examining alternative proposals is required. This is what cost-effectiveness analysis attempts to do. It can be and is applied at any level. For example, the government may require a study examining the implications of the different allocations of national resources to defence, foreign aid, civil industrial research, social security benefits, etc.; the Ministry of Defence may require a study examining the balance between the forces on land, at sea or in the air (force structure) and the contribution each make in a number of scenarios; a single service may require a study of the resources that should be allocated to defensive or offensive action; within a single role a study may be required of the alternative weapons that could be deployed to meet a particular requirement. In this paper, only questions arising in the study of defence systems will be considered.