Elementary general music teachers typically teach hundreds of students every week. Each child has individual learning needs due to a variety of factors, such as prior experiences with music, music aptitude, learning style, and personality. The purpose of this qualitative study was to explore ways that experienced teachers used assessments to differentiate instruction so they could meet the music learning needs of individual students. The guiding questions were as follows: (1) When and how did the participants assess musical skills and behaviors? (2) How did participants score or keep track of what students knew and could do in music? And (3) What was the impact of assessment on differentiation of instruction? I selected three elementary music teachers who had been teaching for at least eight years and were known to use assessments regularly. I observed the first participant as she taught a kindergarten and a fourth grade class every time they met for seven weeks. With the second participant, I observed a third grade, a fourth grade, and a self-contained class for children with cognitive impairments each time they met for four weeks. I observed the final participant each time she taught one first grade and one third grade for seven weeks. In addition to my field notes of these observations, data collection included interviews, teacher journals, videotape review forms, and verbal protocol analysis (think-alouds). Data were analyzed on an ongoing basis using the constant comparative method of data analysis, guided by my initial research questions and also seeking emergent themes. The results are presented in the form of case studies of each teacher's practices, followed by cross-case analysis. All participants used a variety of assessment methods, including rating scales, checklists, report cards, observation, and aptitude testing. Two participants included self-assessments, and one compiled all written work into a portfolio for each student. Although each teacher occasionally assessed specifically for report card grades, most assessment was consistent and ongoing throughout the school year and its primary purpose was to inform instruction. Participants reported that the number of students they taught, lack of time and support, and preparation for performances were major hindrances to assessment, yet they nevertheless each continued consistently to integrate assessment. They disagreed about the role of large-group performance (i.e., after-school "programs" or concerts) as an assessment activity. Although some assessments were directly applied to personalize instruction in a linear or spiraling fashion, assessment practices and differentiation of instruction were typically interwoven in a complex relationship that varied among participants. Group work--including praxial group work, creative group work, and centers-based instruction--was one way that teachers individualized instruction and also assessed the music learning of individual students. Participants utilized a variety of presentation styles and offered a range of musical activities in order to personalize whole-group instruction, as well as providing opportunities for individual responses to open-ended high-challenge and self-challenge activities in whole-group contexts. Furthermore, each participant was expected to differentiate music instruction for students with a variety of special needs. This study concludes with a discussion of the implications of these results for practice and suggestions for future research. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]