An overground rehabilitative powered orthotic system including an exoskeleton comprising: a torso portion configurable to be coupled to an upper body of a person; at least one leg support configurable to be coupled to a first lower limb of the person, with the at least one leg support including at least a thigh link rotatably connectable to the torso portion at a hip joint, and a shank link rotatably connectable to the thigh link at a knee joint; a first actuator for controlling motion of said hip joint; a second actuator for controlling motion of said knee joint; a plurality of sensors for monitoring the exoskeleton; and characterised by: a controller configured to control the first and second actuators (135, 140) with adaptive, variable levels of assistance in response to signals from the plurality of sensors by: initially establishing the powered orthotic system to perform gait functions for the person based on a predetermined level of assistance; and adaptively varying the predetermined level of assistance based on a parameter of the person; and wherein the controller is configured to: a) define a desired assistance trajectory, (b) define one or more segments along said desired assistance trajectory, and (c) assign to each of said one or more segments a function proportional to a distance from said desired assistance trajectory; or (a) determine a previous position along a trajectory, (b) determine a current position along said trajectory, (c) determine a rate of progress along said trajectory, and (d) estimate, from said previous position, said current position and said rate, a future trajectory a torso portion configurable to be coupled to an upper body of a person; at least one leg support configurable to be coupled to a first lower limb of the person, with the at least one leg support including at least a thigh link rotatably connectable to the torso portion at a hip joint, and a shank link rotatably connectable to the thigh link at a knee joint; a first actuator for controlling motion of said hip joint; a second actuator for controlling motion of said knee joint; a plurality of sensors for monitoring the exoskeleton; and characterised by: a controller configured to control the first and second actuators (135, 140) with adaptive, variable levels of assistance in response to signals from the plurality of sensors by: initially establishing the powered orthotic system to perform gait functions for the person based on a predetermined level of assistance; and adaptively varying the predetermined level of assistance based on a parameter of the person; and wherein the controller is configured to: a) define a desired assistance trajectory, (b) define one or more segments along said desired assistance trajectory, and (c) assign to each of said one or more segments a function proportional to a distance from said desired assistance trajectory; or (a) determine a previous position along a trajectory, (b) determine a current position along said trajectory, (c) determine a rate of progress along said trajectory, and (d) estimate, from said previous position, said current position and said rate, a future trajectory initially establishing the powered orthotic system to perform gait functions for the person based on a predetermined level of assistance; and adaptively varying the predetermined level of assistance based on a parameter of the person; and wherein the controller is configured to: a) define a desired assistance trajectory, (b) define one or more segments along said desired assistance trajectory, and (c) assign to each of said one or more segments a function proportional to a distance from said desired assistance trajectory; or (a) determine a previous position along a trajectory, (b) determine a current position along said trajectory, (c) determine a rate of progress along said trajectory, and (d) estimate, from said previous position, said current position and said rate, a future trajectory a) define a desired assistance trajectory, (b) define one or more segments along said desired assistance trajectory, and (c) assign to each of said one or more segments a function proportional to a distance from said desired assistance trajectory; or (a) determine a previous position along a trajectory, (b) determine a current position along said trajectory, (c) determine a rate of progress along said trajectory, and (d) estimate, from said previous position, said current position and said rate, a future trajectory