Two fundamentally different approaches to theory, research, and to the practice of leadership point in opposite and mutually contradictory directions. Because both cannot be valid, a controversy has arisen as to whether leadership should be conceived of as being contingent upon the situation or whether there is one most effective style for all situations. In the first case, the leader changes behavior to fit the situation. In the second, the leader changes the situation to bring it into line with sound principles of behavior as these are emerging in the behavioral sciences. This article has examined the roots this controversy and the widely unexamined assumptions embedded within the conceptual structure supporting each side of its. Eight interrelated kinds of evidence have been presented that lead to the conclusion that one best style is a sounder basis for the exercise of effective leadership-to wit: These findings provide for the resolution of this controversy. They consistently demonstrate that the 9,9 oritentation (System 4, Model II, Theory Y) is the most effective style at the level of conceptual strategy, with tactics of practice geared to situations. This clarification makes it possible to strengthen leadership in its many applications within industry and government, mental health settings, and the academic world by bringing leadership practices into alignment with sound leadership theory.