The aim of this paper is the introduction of preemption in a compositional model, called M-nets, which is based on Petri nets and hence provided with a concurrent semantics. We propose a way to model preemptible systems by extending the M-net model with priorities and the M-net algebra with a preemption operator. We show that these extensions can be seen as a high-level version of the well studied model of priority systems, and so, can be reduced to Petri nets (without priorities) which retain as much as possible of the original concurrency. As a consequence, Petri nets appear as a model powerful enough to deal with preemption in a compositional way and with a concurrent semantics.