The relationship between law and social work is complex. Does law assist practitioners to promote rights, or is it confined to ensuring accountability and regulating practice? This article explores law's role, and its limitations, in promoting liberty, or more accurately ‘constitutional liberty’. Does law provide practitioners with liberty-based values capable, if necessary, of legal enforcement? The paper argues that there is no simple answer; law impacts on social work in different ways and there is little consistency across and within client groups. However, it concludes that law is one of the tools that social workers can use to promote liberty.