We introduce a novel algorithm for transforming character anima- tion sequences that preserves essential physical properties of the motion. By using the spacetime constraints dynamics formulation our algorithm maintains realism of the original motion sequence without sacrificing full user control of the editing process. In contrast to most physically based animation techniques that synthesize motion from scratch, we take the approach of motion transformationas the underlying paradigm for generating computer animations. In doing so, we combine the expressive richness of an input animation sequence with the controllability of spacetime op- timization to create a wide range of realistic character animations. The spacetime dynamics formulation also allows editing of intu- itive, high-level motion concepts such as the time and placement of footprints, length and mass of various extremities, number of body joints and gravity. Our algorithm is well suited for the reuse of highly-detailed cap- tured motion animations. In addition, we describe a new methodol- ogy for mapping a motion between characters with drastically dif- ferent numbers of degrees of freedom. We use this method to re- duce the complexity of the spacetime optimization problems. Fur- thermore, our approach provides a paradigm for controlling com- plex dynamic and kinematic systems with simpler ones.