The Resource Description Framework RDF allows you to describe web documents and resources from the real world - people, organisations, things - in a computer-processable way. Publishing such descriptions on the web creates the semantic web. URIs are very important as the link between RDF and the web. This article presents guidelines for their effective use. We discuss two strategies, called 303 URIs and hash URIs. We give pointers to several web sites that use these solutions, and briefly discuss why several other proposals have problems.