This article reports on the development and validation of the Instrumental Practice Goal Realization Inventory (IPGRI) designed to assess the process of self-directed study, beginning with setting the intention to practice and ending with the evaluation of one's performance. This new tool is based on the theoretical model of action phases. The IPGRI consists of 12 items and four scales delineating the phases of goal realization: Practice Intention, Planning, Acting, and Evaluating Practice. We describe the conceptualization and development of the measure. The evaluation of its psychometric properties is based on two samples: 171 piano students and 235 students learning various instruments. They participated in three measurements, each spaced two weeks apart. Exploratory and confirmatory factor analyses performed on the two samples indicate that the four-factor structure of the IPGRI is a good representation of the realization of instrumental practice goals. A high reliability (internal consistency and test-retest stability) of the scales was observed. We also provide evidence for convergent, discriminant, construct, and known-group validity. The IPGRI is a short and multidimensional measure with good psychometric properties. Therefore, it may be used both in research and practice in the psychology of music and music education.