A current challenge for project network scholars is to explain “how history matters”; in other words, how past experiences of collaboration between actors in a project network are transformed into an (informal) organizational format that is replicable in future collaborations. How are project networks formed in the first place? By examining a collaboration under formation between two organizations, this article proposes that project networks can be conceptualized as psychological contracts. In this formation process, critical incidents play a key role as they define “items” in the psychological contract; in project network terms, these items define routines for collaboration.