Abstract: The rapid development of complex virtual worlds (most notably, in 3D computer and video games) introduces new challenges for the creation of virtual agents, controlled by artificial intelligence (AI) systems. Two important subproblems in this topic area which need to be addressed are (a) believability and (b) effectiveness of agents' behavior, i.e. human-likeness of the characters and high ability to achieving their own goals. In this paper, we study current approaches to believability and effectiveness of AI behavior in virtual worlds. We examine the concepts of believability and effectiveness and analyze several successful attempts to address these challenges. In conclusion, we suggest that believable and effective behavior can be achieved through learning behavioral patterns from observation with subsequent automatic selection of winning acting strategies.