Open Source, in which the source code to software is freely shared and improved upon, has recently risen to prominence as an alternative to the more usual closed approach to software development. A number of high profile projects, such as the Linux operating system kernel and the Apache web server, have demonstrated that Open Source can be technically effective, and companies such as Cygnus Solutions (now owned by Red Hat) and Zope Corporation have demonstrated that it is possible to build successful companies around open source software. Open Source could have significant benefits for hydroinformatics, encouraging widespread interoperability and rapid development. In this paper we present a brief history of Open Source, a summary of some reasons for its effectiveness, and we explore how and why Open Source is of particular interest in the field of hydroinformatics. We argue that for technical, scientific and business reasons, Open Source has a lot to offer.