In this chapter we examine ways of programmatically expanding the functionality of conventional browsers. We consider various mechanisms for creating extensions and add-ons in Internet Explorer and Firefox and briefly review other browsers. We explore some existing browser extensions. The focus is on accessibility extensions, but some prominent other examples of browser extensions are also mentioned. Information for using and creating various types of extensions is presented. Finally, the usefulness of browser extensions compared with other accessibility enablement options for the Web is considered.