Conflict between groups–whether between boys’ gangs, social classes, “races,” or nations–has no simple cause, nor is mankind yet in sight of a cure. It is often rooted deep in personal, social, economic, religious, and historical forces. Nevertheless, it is possible to identify certain general factors which have a crucial influence on the attitude of any group toward others. Intergroup conflict and harmony is not a subject that lends itself easily to laboratory experiments. But in recent years there has been a beginning of attempts to investigate the problem under controlled yet lifelike conditions, and this article includes the results of a program of experimental studies of groups that the author began in 1948. The study was conducted with groups of the informal type, where group organization and attitudes would evolve naturally and spontaneously, without formal direction or external pressures. The aim was to study the development of group relations among the subjects (boys about 11 or 12 years old) under carefully controlled conditions, with as little interference as possible form personal neuroses, background influences, or prior experiences. The experiment shows that the possibilities for achieving harmony are greatly enhanced when groups are brought together to work toward common ends–in short, hostility gives way when groups pull together to achieve overriding goals which are real and compelling to all concerned.