The aim of this chapter is to provide sufficient information to enable a reader, new to the subject of Systems Biology, to create and use effectively controlled annotations, using resolvable Identifiers.org Uniform Resource Identifiers (URIs). The text details the underlying requirements that have led to the development of such an identification scheme and infrastructure, the principles that underpin its syntax and the benefits derived through its use. It also places into context the relationship with other standardization efforts, how it differs from other pre-existing identification schemes, recent improvements to the system, as well as those that are planned in the future. Throughout, the reader is provided with explicit examples of use and directed to supplementary information where necessary.