The authors discuss a general and systematic computational scheme of the inverse dynamics of closed-link mechanisms. It is derived by using d'Alembert's principle and obtained without computing the Lagrange Multipliers. To account for the constraints, only the Jacobian matrix of the passive joint angles in terms of actuated ones is required. Given a nonredundant actuator system, this allows a unique representation of the constraints even for complicated multiloop closed-link mechanisms. The inverse dynamics of closed-link mechanisms that contain redundant actuators and their redundancy optimization are also discussed. For a redundant actuation system that contains N/sub r/ redundant actuators, the passive joint angles are represented by N/sub r/+1 independent ways as functions of actuated joints. Using their Jacobian matrices, the actuation redundancy of a closed-link mechanism is parameterized by an N/sub r/-dimensional arbitrary vector in a linear equation. Numerical examples are given to show the computational efficiency of inverse dynamics computation and the potential of closed-link manipulators with actuation redundancy. >