Robotic systems studied in Chap. 6 have their bases fixed, however, in reality many robotic systems have their bases mobile or floating. In the case of a fixed-base robotic system, the base does not influence the dynamics, whereas it significantly influences the dynamics in the case of a floating-base robotic system. Space manipulators and legged robots are examples of floating-base robotic systems. Legged robots find applications in maintenance task of industrial plants, operations in dangerous and emergency environments, surveillance, maneuvering unknown terrains, human care, terrain adaptive vehicles and many more. In the case of legged robots they are either classified based on the number of legs, e.g., biped, quadruped, hexapod, etc., or the way it balances, e.g., statically or dynamically balanced. As reviewed in Chap. 2, legged robots (1) have variable topology, (2) move with high joint accelerations, (3) are dynamically not balanced if Center-of-Mass (COM) moves out of the polygon formed by the support feet, and (4) are under actuated. Hence, objective of achieving stable motion is difficult to decompose into actuator commands. Therefore, control of legged robots is intricate and dynamics plays vital role in achieving stable motion. © 2013, Springer Science+Business Media Dordrecht.