This paper discusses concepts of command for complex endeavors. The traditional concepts of Unity of Command and Unity of Effort are found wanting, the former because there is no single individual in command of a complex endeavor, the latter because there is often no time to develop the comprehensive plan required, and if there is time, it is nevertheless difficult because different organizations have different planning methods and different planning cultures. A new command concept, based on our experience from studies of peace support operations is proposed. It is called Harmony of Efforts and it is a C2 concept for complex endeavors that involve a number of organizations, each of which does what it usually does, and for which there is no one commander with authority to enforce unity of command or unity of effort. Under these circumstances, Harmony of Efforts is all that one can hope for. The command concept of Harmony of Efforts specifies the spirit in which C2 issues should be approached, which is cooperation, the method to be used, which is negotiation, and the substance of C2, i.e., what C2 can usefully be concerned with in complex endeavors, which is the management of interfaces between organizations.