During the long nineteenth century, international relations were conducted between entities of a variety of types, including those described, at the time, as ‘semi-sovereign’. Such polities, from Belgium and the Ionian Islands, to Egypt and the Indian ‘princely’ states, were found throughout the international society of the period, across a range of temporal, spatial and institutional settings. This chapter focuses on this important but often overlooked group of international actors, seeking to explain their characteristics and constitution, from a comparative and historical perspective. It argues that different semi-sovereigns manifested different combinations of four ‘social logics’: law, management, suzerainty and cultural differentiation. Drawing principally on ideas from ‘relational’ theory, the chapter demonstrates how the social relations at the heart of each logic were productive and constitutive of diverse semi-sovereign polities. These relations interacted with each other, with alternative ‘configurations’ of the logics representing pathways to various modes of semi-sovereignty.