Over the last decades, physics-based modeling of musical instruments has seen increased attention. In 2020 and 2021, the Journal of the Acoustical Society of America accepted submissions for a special issue on the modeling of musical instruments. This article is intended as an introduction to the special issue. Our purpose is to discuss the role that modeling plays in the study of musical instruments, the kinds of things one hopes to learn from modeling studies, and how that work informs traditional experimental and theoretical studies of specific instruments. We also describe recent trends in modeling and make some observations about where we think the field is heading. Overall, our goal is to place the articles in the special issue into a context that helps the reader to better understand and appreciate the field.