Vast human and financial resources have been spent in efforts to understand and address street homelessness. Yet, the problem persists. This think piece summarises the findings of a major review exploring the international evidence base on what works to end street homelessness (Mackie et al., 2017). It also reflects on the question: ‘if we know what works, why don’t we do it?’ Informed by more than 500 literature sources and interviews with 11 international experts, it identifies the key principles which appear to improve the likelihood of interventions ending street homelessness. These include: be housing-led, offer person-centred support and choice, take swift action, employ assertive outreach leading to a suitable accommodation offer, ensure services address wider support needs, and collaborate effectively between agencies and across sectors. The article also identifies seven reasons why those responding don’t always do what is known to work. If street homelessness is to be ended then we must address: the lack of settled accommodation, funding challenges, ineffective collaboration and commissioning, the needs of different subgroups, ineligibility of some people for publicly funded support, overly bureaucratic processes, and the need for stronger political will.