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Increasing Modularity of UAV Control Systems using Computer Game Behavior Trees ⇤ ¨ Petter Ogren Swedish Defence Research Agency (FOI), Stockholm, SE-164 90, Sweden In this paper, we argue that the modularity, reusability and complexity of Unmanned Aerial Vehicle (UAV) guidance and control systems might be improved by using a Behavior Tree (BT) architecture. BTs are a particular kind of Hybrid Dynamical Systems (HDS), where the state transitions of the HDS are implicitly encoded in a tree structure, instead of explicitly stated in transition maps. In the gaming industry, BTs have gained a lot of interest, and are now replacing HDS in the control architecture of many automated in-game opponents. Below, we explore the relationship between HDS and BTs. We show that any HDS can be written as a BT and that many common UAV control constructs are quite naturally formulated as BTs. Finally, we discuss the positive implications of making the above mentioned state transitions implicit in the BTs. I. Introduction Guidance and control of UAVs is an active research area,12 and so far the main e↵ort has been on how to make the UAVs do the tasks right, i.e. designing the controllers,345 rather than doing the right tasks, i.e. designing the control architecture. However, when an increasing number of capabilities are added to a UAV system, the problem of how and when to switch, between di↵erent tasks and corresponding controllers, becom