Practice is the process through which musicians improve their performance abilities and increase their level of expertise. Deliberate Practice (DP) is a theory of expertise based on the concept that interindividual differences in the level of proficiency in a specific domain can be mostly explained by interindividual differences in the amount of deliberate practice; despite its popularity, subsequent studies have demonstrated several critical issues in Ericsson’s DP concept, due to its vagueness in definitions, arbitrary measurements of expertise, and inability to account for the possible role of genes. The present project aimed at creating a new questionnaire, capable of measuring practice quality in terms of deliberate practice for the music domain, regardless of the instrument and musical genre played, at any level of expertise. Based on data from a sample of 1,558 musicians, ranging from amateurs to world-renowned soloists, the Deliberate Practice in Music Inventory (DPMI) was created, a self-report questionnaire and measurement instrument for practice quality consisting of a main DP scale and four subscales: Process improvement, Practice competences, Mindless practice (inverted scale), and Task decomposition. Results indicated that musicians who implement effective practice habits are focused on solving problems related to music playing and often refine their practice routines to increase their effectiveness. In addition, musicians who usually exhibit high amounts of DP behavior often decompose long and complex tasks into shorter and simpler elements, aiming to master them more easily and in shorter time. The DPMI instrument shows good convergent validity with measures related to expertise in music as well as good predictive validity for performance improvement. The DPMI generates new perspectives for the field of musical expertise research.