In competitive and evolving environments only organizations which can manage complexity and can respond to rapid change in an informed manner can gain a competitive advantage During the early 90's, workflow technologies offered a transversal integration capacity to the enterprise applications. Today, to "integrate" enterprise applications -and the activities they support- into business processes is not sufficient. The architecture of this integration should also be flexible. Enterprise requirements highlight flexible and adaptive processes whose execution can evolve (i) according to situations that cannot always be prescribed, and/or (ii) according to business changes (organizational, process improvement, strategic ...). More recent works highlight requirements in term of flexible and adaptive workflows, whose execution can evolve according to situations that cannot always be prescribed. This paper presents the state of the art for flexible business process management systems and criteria for comparing them.