The purpose of this qualitative summative content analysis was to explore the theoretical foundations that inform music therapists’ work with families. An additional aim was to see if music therapists are developing or incorporating theories that are unique to music therapy to inform their work. A review of the literature indicated that music therapists are using many different theories to support their work with families. These theories also came from different fields including psychoanalysis, child development, neurology, and music therapy. Qualitative and quantitative music therapy studies were systematically reviewed for theoretical principles and assumptions. The student-researcher also explored how these assumptions related to the music therapists’ approach and evaluation or assessment tools. The analysis indicated that music therapists are actively developing and refining theoretical premises and assumptions to better understand their work with families. The student-researcher identified three ways that theory was used to inform this work: First, music therapy researchers are increasingly using the theory of communicative musicality to support their work. Second, music therapy researchers have used theory to inform the development of family-centered approaches. Third, researchers are using theories to develop music-therapy-specific evaluation and assessment tools. These findings suggest that theoretical development in this area is ongoing and the student-researcher hopes that this inquiry can support a continued development and refinement of the theories that inform music therapists’ work with families.