A key focus of homelessness policy across the Anglophone world is to prevent homelessness by targeting interventions to those considered “at-risk”, yet the term “at-risk of homelessness” remains ambiguous. A solid definition is required. Typically, risk is defined using those factors that are over-represented in the population of interest. However, this approach tends to obscure the relationship between risk factors and their broader causes. It is also unfeasible given the characteristics of the current body of knowledge on homelessness. This paper takes a different approach. It argues that clarifying what is meant by “at-risk of homelessness” requires understanding the type(s) of causation involved, the various causes of homelessness and the mechanisms through which they act. The paper addresses each of these three requirements to propose a provisional definition of “at-risk of homelessness”. Following further empirical work, this definition can be used to enumerate the “at-risk of homelessness” population and design prevention strategies.