Body-weight-supported locomotor training is an activity-based therapy used frequently to train individuals with spinal cord injury (SCI) for restoring walking ability. Locomotor training after SCI is developed on the basic scientific findings of activity-dependent neuroplasticity. Based on the research from animal SCI models, there exists a spinal neural networks for locomotion which can be reactivated by intense repetitive locomotor training. Notably, the effectiveness of locomotor training depends largely on the severity of injury and time after injury. Locomotor training, using body-weight-supported walking overground or on a treadmill, with assistance manually or robotically, with a variety of training intensity and training programs, has been shown to elicit improvements in locomotor function for incomplete SCI individuals. For chronic and motor complete SCI, other interventions with proven effectiveness such as epidural stimulation might be applied in addition to locomotor training to improve the chance of locomotor recovery. In this chapter, we review the factors that influence the functional outcomes of locomotor training. We also summarize the circuitry, cellular and molecular levels of mechanisms underlying the positive role of locomotor training in inducing neuroplasticity and functional recovery following SCI.