Each paper in an American Society of Zoologists' symposium on territoriality generated new ideas for future research. However, since similar themes repeatedly arose, an overview of the symposium revealed the areas in which future research is most needed. Theory far outstrips data on territoriality at this point, and efforts should concentrate on field tests of ideas. Some of the general areas that need study are: the effect of foraging goals and constraints on territorial decisions, the effect of the internal energy or breeding state of the animal in making territorial decisions, the effects of subtle factors such as habitat geometry and the role of neighbors on the cost of defense, the effect of various forms of environmental variability on territorial decisions, the active or passive influence of plants on territoriality, and the effect of scale and mobility in the expression and evolution of territoriality. The study of territoriality is entering a particularly challenging stage of development, because study systems must be carefully chosen and model assumptions thoroughly examined before rigorous experimental tests of hypotheses can be performed. Yet the state of the art is such now that only rigorous tests will advance our knowledge further.