Critical illness has many devastating sequelae, including profound neuromuscular weakness and psychological and cognitive disturbances that frequently result in long-term functional impairments. Early rehabilitation begun in the intensive care unit (ICU) is emerging as an important strategy both to prevent and to treat ICU-acquired weakness, in an effort to facilitate and improve long-term recovery. Rehabilitation may begin with range of motion and bed mobility exercise, then may progress when the patient is fully alert and able to participate actively to include sitting and posture-based exercise, bed to chair transfers, strength and endurance exercises, and ambulation. Electrical muscle stimulation and inspiratory muscle training are additional techniques that may be employed. Studies conducted to date suggest that such ICU-based rehabilitation is feasible, safe, and effective for carefully selected patients. Further research is needed to identify the optimal patient candidates and procedures and for providing rehabilitation in the ICU.