Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for event-driven module loading. In one aspect, a method includes obtaining a mapping for a resource, wherein the mapping identifies one or more events that, upon occurrence, will trigger a dynamic loading of one or more code modules that are referenced by the resource, determining that an event has occurred, identifying one or more of the code modules that are associated with the event, based on the mapping, and dynamically loading the identified code modules.