The Homelessness Outcomes Star (HOS) is probably the most widespread form of outcome measurement employed by homelessness service providers. Developed in the UK, the HOS is now being used by homelessness services in other European countries and Australia, while being promoted internationally as a validated set of key performance indicators. This paper examines the ideological framework that underpins the HOS, as well as the theoretical and methodological approaches that inform its operation. The review concludes that while there is some utility in the measurement of relative progress for individual service users, the HOS has important limits, both as a means of comparative outcome analysis and as a validated measure of homelessness service outcomes