The decision problem of choosing between several competing goals, when each has conflicting characteristics, time is limited, and the environment is dynamic. Action selection, which is a topic in artificial intelligence, may seem to be a technical issue but is in fact a deep and fundamental problem faced by all agents in complex environments. Animals, mobile robots, and software agents must all respond to changing events in their worlds and they must opportunistically decide which goals are best to pursue, and when to interrupt action towards current goals and divert resources to new goals.