Abstract: This paper examines the long-term and spillover effects of management interventions on firm performance. Under the Training Within Industry (TWI) program, the U.S. government provided management training to firms involved in war production between 1940 and 1945. Using a newly collected panel dataset on all 11,575 U.S. firms that applied to the program, we find that the TWI training had positive and long-lasting effects on firm performance and the adoption of beneficial managerial practices. Moreover, it generated complementarities among different types of training and had positive spillover effects on the supply chain of trained firms.