Software is rapidly moving from the desktop to the Web. The Web provides a generic user interface that allows ubiquitous access, instant collaboration, integration with other online services, and avoids installation and configuration on desktop computers. For software development, the Web presents a shift away from developer workstations as a silo, and has the promise of closer collaboration and improved feedback through innovations in Web-based interactive development environments (IDEs). Moving IDEs to the Web is not just a matter of porting desktop IDEs; a fundamental reconsideration of the IDE architecture is necessary in order to realize the full potential that the combination of modern IDEs and the Web can offer. This paper discusses research challenges and opportunities in this area, guided by a pilot study of a web IDE implementation.