“Sounds good, but… what is it?” This is a common reaction to outcome measurement by music therapy practitioners and researchers who are less familiar with its meanings and practices. Given the prevailing evidence-based practice movement, outcome measurement does ‘sound good’. Some practitioners and researchers, however, have a limited or unclear understanding of what outcome measurement includes; particularly with respect to outcome measures and related terminology around their use. Responding to the “what is it?” question, this article provides an introduction to such terminology. It explores what outcome measures are and outlines characteristics related to their forms, uses and selection criteria. While pointing to some debates regarding outcome measurement, including its philosophical underpinnings, this introduction seeks to offer a useful platform for a critical and contextual understanding of the potential use of outcome measures in music therapy