Music is a good tool to communicate with children no matter what kinds of disorders they have because therapists can interact with them via music and musical instruments without using a single word. Verbal and physical abilities are limited in patients who are physically, cognitively, and socially disabled. Babies also have a similar limitation. Baby science is interdisciplinary research investigating how infants understand and communicate with the surrounding world. Therefore, medical music therapy based on baby science (baby-science-based music therapy) is useful in the rehabilitation of patients with various neuromuscular and developmental disorders to maintain, restore, and develop their physical, cognitive, social, and communicative abilities. Basic concepts and their application methods are as follows: imitation (neonatal oral imitation and higher imitation), integration of crossmodal sensation, rhythm, language, attention, and habituation and dishabituation. To improve the quality of life of patients with severe motor disability and massive involuntary movements, a special music keyboard connected to switch devices (micro switches for Duchenne muscular dystrophy and spastic cerebral palsy, and push-button switches for Rett syndrome and athetoid cerebral palsy) has been developed. This assistive technology enables patients to play music and participate in social activities even in the advanced stage of their diseases.