Music therapy is the clinical and evidence-based use of music interventions to accomplish individualized goals within a therapeutic relationship by a qualified professional who has completed an approved music therapy program.
In other words, music therapy is the use of music by a trained professional to achieve therapeutic goals. Goal areas may include, but are not limited to, motor skills, social/interpersonal development, cognitive development, self-awareness, and spiritual enhancement.
Music therapists are found in nearly every area of the helping professions. Some commonly found practices include developmental work (communication, motor skills, etc.) with individuals with special needs, songwriting and listening in reminiscence/orientation work with the elderly, processing and relaxation work, and rhythmic entrainment for physical rehabilitation in stroke victims.
The idea of music as a healing modality dates back to the beginnings of history, and some of the earliest notable mentions in Western history are found in the writings of ancient Greek philosophers. Robert Burton wrote in the 17th century in his classic work, The Anatomy of Melancholy, that music and dance were critical in treating mental illness, especially melancholia.   
Music Therapy is considered one of the expressive therapies.
In its current/modern form has existed in the United States since around 1944, when the first undergraduate degree program in the world was founded at Michigan State University and the first graduate degree program at The University of Kansas.
Music Therapy in the United States
The American Music Therapy Association (AMTA) was founded in 1998 as a merger between the National Association for Music Therapy (NAMT, founded in 1950) and the American Association for Music Therapy (AAMT, founded in 1971). Numerous other national and international organizations exist, such as the Nordoff-Robbins Center For Music Therapy and The Bonny Institute. In the United States, a music therapist is most commonly designated by MT-BC (Music Therapist, Board-Certified). A music therapist with only this designation has a bachelor's in music therapy and is trained in the specific use of music therapy techniques as an adjunctive/augmentative therapy, complementing the work of other practitioners from different disciplines such as social work, speech/language, physical therapy, medicine, nursing, education, and so forth. A music therapist may have different credentials or professional licenses and may also have a master's degree in music therapy or in another clinical field (social work, mental health counseling, or the like). Some practicing music therapists have held Ph.D.s in non-music-therapy (but related) areas, but more recently Temple University founded a true music therapy Ph.D. program. A music therapist will typically practice in a manner that incorporates music therapy techniques with broader clinical practices such as assessment, diagnosis, psychotherapy, rehabilitation, and other practices. Music therapy services rendered within the context of a social service, educational, or health care agency are reimbursable by insurance and sources of funding for individuals with certain needs. Music therapy services have been identified as reimbursable under Medicaid, Medicare, Private insurance plans and other services such as state departments and government programs.
A U.S. music therapist may also hold the designation of CMT, ACMT, or RMT--initials which were previously conferred by the now-defunct AAMT and NAMT. More current music therapists hold the designation, MT-BC, music therapist-board certified. In Canada, the designation is MTA (Music Therapist Accredited/Musicotherapie Accreditee).
A degree in music therapy requires proficiency in guitar, piano, voice, music theory, music history, reading music, improvisation, as well as varying levels of skill in assessment, documentation, and other counseling and health care skills depending on the focus of the particular university's program. To become board-certified in the United States, a music therapist must complete 1200 hours of clinical training in addition to required coursework, research, and passing a nationally accredited certification exam. They also need to be able to deal with children and adults with disabilities
Music therapy in the United Kingdom
Live music was used in hospitals after both of the World Wars, as part of the regime for some recovering soldiers. Clinical Music therapy in Britain as it is understood today was pioneered in the 60s and 70s by French cellist Juliette Alvin, whose influence on the current generation of British music therapy lecturers remains strong. The Nordoff-Robbins approach to music therapy developed from the work of Paul Nordoff and Clive Robbins in the 1950/60s. It is grounded in the belief that everyone can respond to music, no matter how ill or disabled. The unique qualities of music as therapy can enhance communication, support change, and enable people to live more resourcefully and creatively. Nordoff-Robbins now run music therapy sessions throughout the UK, US, South Africa, Australia and Germany. Its head quarters are in London where it also provides training and further education programmes, including the only PHD course in music therapy available in the UK. Music therapists, many of whom work with an improvisatory model (see clinical improvisation), are active particularly in the fields of child and adult learning disability, but also in psychiatry and forensic psychiatry, geriatrics, palliative care and other areas. Practitioners are registered with the Health Professions Council  and from 2007 new regisrants must normally hold a masters degree in music therapy. There are masters level programmes in music therapy in Bristol, Cambridge, Cardiff, Edinburgh and London, and there are therapists throughout the United Kingdom. The professional body in the UK is the Association of Professional Music Therapists  while the British Society for Music Therapy  is a charity providing information about music therapy.
Important Music Therapists and writers on Music Therapy
- Juliette Alvin
- Kenneth Bruscia
- Leslie Bunt
- Rajasree Mukherjee - see Rajasree
- Paul Nordoff - see Nordoff-Robbins
- Mercedes Pavlicevic
- Mary Priestley
- Clive Robbins - see Nordoff-Robbins
- Celine Dewy
- E. Thayer Gaston
- Michael H. Thaut
- Concetta M. Tomaino - see Institute for Music and Neurologic Function
- Lisa Summer
- Helen Bonny
- Tony Wigram
- Jeff Engel
- cf. The Anatomy of Melancholy, Robert Burton, subsection 3, on and after line 3480, "Music a Remedy":
But to leave all declamatory speeches in praise of divine music, I will confine myself to my proper subject: besides that excellent power it hath to expel many other diseases, it is a sovereign remedy against  despair and melancholy, and will drive away the devil himself. Canus, a Rhodian fiddler, in Philostratus, when Apollonius was inquisitive to know what he could do with his pipe, told him, "That he would make a melancholy man merry, and him that was merry much merrier than before, a lover more enamoured, a religious man more devout." Ismenias the Theban, Chiron the centaur, is said to have cured this and many other diseases by music alone: as now they do those, saith Bodine, that are
troubled with St. Vitus's Bedlam dance. 
- "Humanities are the Hormones: A Tarantella Comes to Newfoundland. What should we do about it?" by Dr. John Crellin, MUNMED, newsletter of the Faculty of Medicine, Memorial University of Newfoundland, 1996.
- Aung, Steven K.H., Lee, Mathew H.M., "Music, Sounds, Medicine, and Meditation: An Integrative Approach to the Healing Arts", Alternative & Complementary Therapies, Oct 2004, Vol. 10, No. 5: 266-270. 
- Dr. Michael J. Crawford page at Imperial College London, Faculty of Medicine, Department of Psychological Medicine.
Crawford, Mike J. (2006). "Music therapy for in-patients with schizophrenia: Exploratory randomised controlled trial". The British Journal of Psychiatry (2006). 189: 405–409. Unknown parameter
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- Gold, C., Heldal, T.O., Dahle, T., Wigram, T., "Music therapy for schizophrenia or schizophrenia-like illnesses", Cochrane Database of Systematic Reviews, 2006, Issue 4.
- http://www.musictherapy.org The American Music Therapy Association
- http://www.musictherapy.ca Canadian Association for Music Therapy
- http://www.cbmt.org Certification Board for Music Therapists
- http://www.bsmt.org/ British Society for Music Therapy
- http://www.apmt.org/ Association of Professional Music Therapists (United Kingdom)
- http://www.njmt.no Nordic Jounal of Music Therapy