The paper focuses on the impact of managerial efficiency on output. Three sources of managerial efficiency are identified: (a) superior initial managerial endowments, (b) the accumulation of managerial knowledge and skills through learning and (c) the impact of an effective market for managerial resources internal to the firm. All three are explicitly measured by appropriate variables and their impact is examined in the context of variously specified production functions. The empirical analysis is carried out with data for approximately 5,000 new manufacturing plants in the United States over the 1973-92 period. It is found that variation in managerial endowments is an important explanatory variable for output with all other relevant inputs controlled. It is further found that the survival of plants with superior managerial efficiency, and the death of those with inferior efficiency, explains a substantial fraction of total factor productivity change in the manufacturing sector of the U.S. economy. There is also clear evidence of the significance for efficiency of internal markets as well as evidence of learning as plants age. Learning and superior managerial resources of old plants largely offset the benefits of capital goods of later vintage of new plants.