A control problem involving a mechanical system with generalized coordinates q¿Rm is considered. The error in tracking a desired input yd ¿ RP is e = E(q,yd) ¿ Rm. If E satisfies simple conditions, it leads to a nonlinear control law which assures e(t) ¿ 0 as t ¿ ¿. The law is robust in that small changes in it do not produce large steady-state errors or loss of stability. The theory presents a unified framework for treating a number of problems in the control of mechanical manipulators. This is illustrated by an example.