Teamwork is common in the social interactions but is not restricted to humans. Animals, from ants to whales, also work in teams. This chapter focuses on working as a team in fields such as robotics, management, and sociobiology. The chapter explains the structural organization of tasks, that is, a classification of task types and examines teamwork in insect societies, in other (non-human) animal groups, in robotics, and in humans. The fundamental task types based upon the interrelationship between subtask types are individual task, group task, partitioned task, and team task. This classification helps to rank different tasks from different systems objectively, to correlate a particular task's complexity with organization size or evolutionary history, or to follow how a particular task is tackled in a more complex, collaborative manner than at other times. The chapter draws attention to a number of misconceptions about teamwork, objectively and rigorously tests for teamwork, and distinguishes it from related phenomena such as group work.