The effective functioning of social systems such as the police service is often seen to be dependent upon the quality of its leadership. If subjective judgments concerning leadership potential could be replaced with a valid test of leadership, many costly errors could be avoided. Senior officers often attempt to identify 'born leaders' but are seldom successful. In the field of personality theory, the search for personality traits that distinguish leaders from nonleaders has led to the realization that leadership might be expressed as a set of highly specific traits which are situation specific; leaders for one situation may have a different set of traits than leaders who must perform a different role, and the leader for all situations probably does not exist. The stereotype that is often sought by senior officers -- the swashbuckling, charismatic, born leader -- is rarely found and would probably be unacceptable, in reality, to most departments. Research into the behavior patterns which characterize effective leaders has suggested that 'consideration' and 'initiating structure' are the two most significant characteristics of leaders. Consideration includes mutual trust, respect, and warmth; initiating structure includes the abilities to organize, to define group activities, and to supervise. Studies of situational factors related to effective leadership have suggested that the leadership situation is more responsive to alteration than is the individual's basic leadership style. Persons in leadership positions should be taught to recognize the conditions under which they can best perform and to modify situations to fit their leadership styles. Numerous studies are reviewed and 29 references are included.