There is an expanding field of research into how making or listening to music can improve wellbeing. As a spontaneous, social, creative nonverbal process unfolding in real time, musical improvisation between individuals is a unique psychological phenomenon distinct from other areas of musical activity. It may therefore have an influence on health or wellbeing distinct from other musical behaviours, and from other components of a musical intervention. Given the psychological complexity of this behaviour it is important to establish the parameters of improvisation, the effects on health or wellbeing that are perceived or claimed for it, and any mechanisms understood to bring about these effects. To establish this, literature was reviewed that explicitly investigates or theorises about the capacity of musical improvisation to influence health or wellbeing. Only work examining its application within music therapy was identified. The behaviours and interactions that constitute improvisation during music therapy are clearly defined. Improvisation in music therapy is seen to have specific benefits for particular populations including the amelioration of neurological damage, improvements in mental health conditions, reductions in stress and anxiety, and improved communication and joint attention behaviours in children with autistic spectrum disorders. Four unique characteristics of musical improvisation are identified as underlying these effects: its potential to link conscious with unconscious processes, the demands on attention of absorption in a creative process, the non-verbal social and creative interaction experienced, and the capacity for expressing difficult or repressed emotions without having to articulate these verbally. Although improvisation is undertaken in music therapy for a purpose distinct from that of improvisation in other contexts, its processes can be seen as substantively similar, suggesting that improvising in itself may offer intrinsic benefits to health or wellbeing to broader populations and outwith the therapeutic context. Based on this review, a model is proposed for how improvisation in music can influence the health or wellbeing of those involved.