A methodology for vulnerability assessment of air force bases subject to armed attack, primarily with fragmenting warheads, is presented. The methodology may be useful also when analysing other large installations, such as harbours. The proposed methodology describe a procedure including assessment of the vulnerability of single objects up to a complete airbase. A number of unique difficulties arise during assessments of complex and geographically dispersed system of systems, as compared to the assessment of a solitary weapons platform. First, which aim point or aim points should the aggressors' weapons be given, as the military intelligence status of the aggressor is unknown. Assessing which capabilities that are lost, and those available after an attack, requires a high level of understanding of how different subsystems interact and depend on each other. It is relatively straight forward to make an estimate of the number of injured/incapacitated fighter pilots or damaged objects, but the end result should also describe the base-unit's ability to conduct their primary missions after being attacked. The amount of data generated also leads to difficulties in the selection of results to be presented. To obtain the aims and goals with an assessment it is important with a good dialogue between the customer and the provider. One aim with this report is to give both parties a common view of the methodology and the needs for information. The report concludes that in order to make vulnerability assessments of airbases possible in a rational way, the tools being used needs to be further development and a library of unitary objects present on a base, for instance vehicles and buildings, needs to be created and maintained. To make an assessment of the vulnerability of an airbase already during the design phase gives important knowledge concerning how the base should be designed. Also, the already modelled parts of the airbase can be reused later when changes in the threat environment requires new evaluations.