The distinction between abstract and concrete entities, especially its precise characterization, remains underexplored in foundational ontology research. This paper aims to constitute the initial steps towards a formal ontology of abstracta and concreta. We begin by presenting three existing criteria (epistemic, spatiotemporal and causal) for the abstract/concrete distinction. We illustrate them with some well-known upper ontologies. After examining the shared assumption by the three criteria that any entity is either abstract or concrete but not both, we develop an alternative and more general framework for formally representing abstract and concrete entities. In particular, we propose a relational account of them by introducing the relation of “concretization”. The pivotal idea is that being abstract (or concrete) amounts to being concretized by (or concretizing) some other entity. We also briefly discuss a concretization-based reinterpretation of the spatiotemporal criterion and universals.