From its earliest inception, FOLIO was conceived not as an ILS (Integrated Library System), but as a true Services Platform, composed of many independent but interdependent modules, and forming a foundation on which an ILS or other library software could be built out of relevant modules. This vision of modularity is crucial to FOLIO’s appeal to the library community, because it lowers the bar to participation: individual libraries may create modules that meet their needs, or hire developers to do so, or contribute to funding modules that will be of use to a broader community — all without needing “permission” from a central authority. The technical design of FOLIO is deeply influenced by the requirements of modularity, with the establishment of standard specifications and an emphasis on machine-readable API descriptions. While FOLIO’s modular design has proved advantageous, it also introduces difficulties, including cross-module searching and data consistency. Some conventions have been established to address these difficulties, and others are in the process of crystallizing. As the ILS built on FOLIO’s platform grows and matures, and as other application suites are built on it, it remains crucial to resist the shortcuts that monolithic systems can benefit from, and retain the vision of modularity that has so successfully brought FOLIO this far.