At the present, the Swedish Air Force is integrating the new JAS 39 Gripen E into its fleet of aircraft, as well as procuring new medium range air-to-air missiles. Within the next few years, modifications of the currently used air-to-air missiles will likely take place, and Sweden will have to decide what the next step beyond JAS 39 Gripen will be. All of these changes are deemed to increase the support required by the Swedish Armed Forces in questions regarding the air domain. In order to meet the increased need for support, it is vital that FOI continues to develop methods and tools, in particular for the purpose of systems assessment and as support for future technical development and procurement. Earlier work at FOI shows that the tools for simulating air combat with the purpose of combat effectiveness assessment that are currently at FOI's disposal do not fulfil current requirements. In addition, studies have been unable to find a suitable commercial alternative. With this in mind, development of a new simulation software tool has started as part of the research-and-development project Stridseffektvärdering av samverkande vapensystem (SSV). The purpose of this development is to create a tool that better fits FOI's requirements, which is presented in this report: ACSim (Air Combat Simulation). ACSim is envisioned as a tool for air combat simulation with emphasis on air-to-air combat, weapon systems, cooperative engagement and combat effectiveness assessment.