This article describes an approach to motor synergies that allows them to be quantified in people with atypical movement patterns during exercise or practice. Within this approach, motor variability may be classified with respect to a task-specific performance variable as "good" (not affecting the variable) or "bad" (changing the variable). The authors review studies of motor synergies in people with typical movement patterns, in people with Down syndrome, in patients after stroke, and in elderly people. Two stages of practice effects on motor synergies are described as being characterized by different changes in the synergy index: an increase followed by a drop in the index. Synergy changes with practice may be accompanied by plastic changes in both descending projections from the primary cortex and interhemispheric projections. The authors emphasize the importance--for practitioners in the area of motor disorders and rehabilitation--of being aware of the latest progress in motor control and coordination.