Abstract – For conventional designs of robots, manipulator motions result in forces and moments on the base. These forces and moments may cause undesirable translation and rotation of the base. The objective of this paper is to systematically analyze the fundamentals of reactionless robots. Based on this analysis, designs of two distinct classes of spatial robots are proposed. The designs are achieved through appropriate choices of geometric and inertial parameters. Due to the underlying conservation laws, the trajectory must satisfy additional constraints. We illustrate the reactionless feature of these robots through computer simulations. Currently, we are fabricating reactionless robots to illustrate the underlying concepts.