Partially observed actionsare observations of action execu- tions in which we are uncertain about the identity of ob- jects, agents, or locations involved in the actions (e.g., we know that actionmove(?o,?x,?y) occurred, but do not know ?o,?y). Observed-Action Reasoningis the problem of rea- soning about the world state after a sequence of partial obser- vations of actions and states. In this ... [Show full abstract] paper we formalize Observed-Action Reasoning, prove intractability results for current techniques, and find tractable algorithms for STRIPS and other actions. Our new algorithms update a representation of all possible world states (the belief state) in logic using new logical constants for un- known objects. A straightforward application of this idea is incorrect, and we identify and add two key amendments. We also present successful experimental results for our algorithm in Blocks-world domains of varying sizes and in Kriegspiel (partially observable chess). These results are promising for relating sensors with symbols, partial-knowledge games, multi-agent decision making, and AI planning.