This paper aims at classifying and discussing the various ways along which the object paradigm is used in concurrent and distributed contexts. We distinguish the applicative approach, the integrative approach, and the reflective approach. The applicative approach applies object-oriented concepts, as they are, to structure concurrent and distributed systems through libraries. The integrative approach consists in merging concepts such as: object and activity, message passing and transaction, etc. The reflective approach integrates protocol libraries intimately within an object-based programming language. We discuss and illustrate each of these approaches and we point out their complementary levels and goals.