Prosthetic knee units generally consist of the integration of polycentric or fixed-axis joints and frictional or damping elements, whereas prosthetic ankle units generally consist primarily of stiff leaf spring elements. Technological advances in the late 1980s (most prominently, the increasing availability of microcontrollers) enabled the emergence of microprocessor-controlled knee (MPK) units in the mid-1990s. Rather than maintain preset damping characteristics, an MPK incorporates an electrically controllable damping element and uses sensors in combination with a microcontroller to modulate the degree of resistance imposed by the knee unit to better accommodate a given activity. Such units are presumably better able to accommodate activity variation relative to level walking at a given speed, such as walking at different speeds or descending stairs. Just as the increasing availability of microcontrollers in the late 1980s resulted in the subsequent emergence of MPKs, recent advances in robotics technology (including advances in battery capacity, motor design, and microelectronics) are now enabling the emergence of powered prostheses. Like MPKs, powered prostheses incorporate sensors and a microcontroller. Rather than modulating only joint resistance, however, an appropriately designed powered prosthesis can provide a full range of physical behaviors. Like an MPK, a powered prosthesis can modulate the level of joint resistance but can additionally or alternatively modulate the level and character of joint stiffness or provide direct movement of the joint. Alternatively stated, a powered prosthesis is not limited to providing resistance but rather can provide any physical behavior offered by an agonist/antagonist muscle group. As such, although microcontrollers have at present found primary application in knee units for purposes of modulating the degree of knee resistance, powered prostheses are equally applicable to knee or ankle joints. Further, in the case of a transfemoral prosthesis, both joints can be controlled together in a coordinated fashion, much like the human neuromuscular system. In this article, the author presents data illustrating some of the features provided by a transfemoral prosthesis with powered and coordinated knee and ankle joints in various activities, including level walking, slope ascent and descent, stair ascent and descent, and standing on uneven ground. In the Discussion section of this article, the author suggests a number of potential biomechanical benefits that may be provided by powered prostheses, relative to conventional lower-limb prosthesis technology.