A natural obligation (obligatio naturalis) is a legal construction whose roots stretch back to Roman law. This common source means that we will find similar solutions in legal systems descended from Roman legal culture – with respect to both the understanding of natural obligations and specific instances where they arise. The aim of this paper is to answer the question of whether these different systems define natural obligations in the same manner or whether the natural obligations encountered in these systems are distinct legal institutions sharing only a common name. In this paper, the various approaches of contemporary legal systems to this issue are characterized. Then, a comparative-law analysis focuses on three fundamental aspects of natural obligations: their legal construction (definition), a catalogue of instances, and their legal effectiveness. Under the constructional perspective, two basic models of obligatio naturalis are distinguished and discussed – the obligative model and the causal model – and it is around these two models which the particular conceptions converge. The analysis presented in the paper demonstrates that the similarities between the various models outweigh the differences. This permits us to refer to obligatio naturalis as a universal legal construction.