Statelessness is a concept of international law, defined by a UN treaty that was adopted over half a century ago. This chapter focuses on the historic evolution and contemporary understanding of statelessness with a view to identifying overlap, complementarity and divergence. The chapter explores the discourses relating to 'de facto statelessness' and 'ineffective nationality', and positions these within the broader context of the challenge of 'protection' under contemporary human rights law. It focuses on the blurring of conceptual boundaries between statelessness and related phenomena, commenting on the implications of a failure to distinguish statelessness as a distinct issue for the effectiveness of remedial measures. With the underlying aim of supporting and informing real-world efforts to address this global human rights challenge, the chapter expresses that a greater awareness and application of the legal definition of statelessness in related research design, publications and debate is essential.