This thesis explores different aspects of extensibility in computer music software. Extensibility refers to how core developers, third-party developers, and users can extend software. It is a primary factor in determining a software’s range of use cases and capacity to grow over time. This has a direct impact on the robustness of both the software and the user’s work. This thesis discusses four main areas of research: extensibility in programming languages, platform extensibility, run-time modular software development, and music systems as libraries. It also explores these areas through the development of four open-source software projects: Csound, Blue, Pink, and Score. Csound and Blue are existing programs that have been modified to provide new means of extension. Pink and Score are new software libraries designed for extension from the start. The goal of examining extensibility is to help create long-living computer music software and – by extension – enduring musical works. These in turn will hopefully provide future developers, users, and curious students with the means not only to interact with the past through documentation, but also to actively explore, experience, and use these programs and works.