Dynamic control of constrained mechanical systems, such as robotic manipulators under end-effector constraints, parallel manipulators, and multifingered robotic hands under closure constraints have been classic problems in robotics research. There have been numerous treatments on modeling, analysis, and control for each class of problem. In this paper, we provide a unified geometric framework for modeling, analysis, and control of constrained mechanical systems. Starting with the constraint, we define two canonical subspaces, namely the subspace of constraint forces and the tangent space of the constraint manifold for holonomic constraint. Using the kinetic energy metric, we define the remaining subspaces and show explicitly the relations among these subspaces. We project the Euler