CENTRAL TO THE THEORY OF LEADERSHIP EFFECTIVENESS OUTLINED HERE IS A CONTINGENCY MODEL, ACCORDING TO WHICH GROUP EFFECTIVENESS DEPENDS ON INTERACTION BETWEEN LEADERSHIP STYLES AND THE DEGREE TO WHICH SITUATIONS ENABLE LEADERS TO EXERT INFLUENCE. THE THEORY PREDICTS THAT A TASK-ORIENTED STYLE WILL BE THE BEST STYLE IN FAVORABLE LEADERSHIP SITUATIONS, A RELATIONSHIP-ORIENTED STYLE WILL BE BEST IN INTERMEDIATE SITUATIONS, AND A TASK-ORIENTED STYLE WILL AGAIN BE MOST EFFECTIVE IN UNFAVORABLE GROUP SITUATIONS. TWO MEASURES OF LEADERSHIP STYLE--THE LEAST PREFERRED COWORKER (LPC) AND THE ASSUMED SIMILARITY BETWEEN OPPOSITES (ASO)--ARE USED. VALIDATION STUDIES (NOTABLY THE BELGIAN NAVAL STUDY), GROUP PROCESS AND GROUP PERFORMANCE, AND STUDIES EXTENDING THE MODEL TO GROUPS UNDER STRESS, COACTING AND COUNTERACTING GROUPS, AND SECOND AND THIRD LEVEL MANAGERIAL PERSONNEL, ARE DISCUSSED, TOGETHER WITH CRITERIA FOR LEADERSHIP STYLES AND A CLASSIFICATION OF GROUPS AND GROUP TASKS. AREAS FOR FUTURE RESEARCH ARE ALSO SUGGESTED. (THE DOCUMENT INCLUDES INDEXES AND APPENDIXES, NUMEROUS REFERENCES, AND 78 TABLES AND FIGURES.) THIS DOCUMENT IS AVAILABLE FROM THE MCGRAW-HILL BOOK COMPANY, NEW YORK. (LY)