This thesis explores the role of social work in shaping access to social protection among migrants who are undocumented or hold a precarious immigration status, with an empirical focus on Finland. This diverse group of precarious migrants includes both people living in the country without a residence permit and legally residing migrants such as seasonal workers, EU migrants without a right of residency, and international students from outside the European Union. Although these groups are widely excluded from public social protection provision, they remain entitled to ‘indispensable subsistence and care’. In practice, it is often statutory social workers who widely determine what this right entails, as they are mandated to determine access to social services. Moreover, multiple NGOs provide social protection for precarious migrants. Drawing on ethnographic research conducted in social services in the public sector and in NGOs, the thesis shows that social workers support precarious migrants by way of providing access to vital resources and by rejecting police requests to share client data for immigration enforcement. However, despite similar legal entitlements, precarious migrants receive different treatment by social workers depending on how they are categorised in social services, resulting in a differential access to social protection within the diverse group.