Emergence is a systemic phenomenon in an System-of-Systems (SoS) that cannot be reduced to the behavior of the isolated parts of a system. It is the objective of this paper to contribute to the understanding of emergent phenomena in SoSs. After a short look at the literature on emergence in the domains of philosophy and computer science, this paper continues with an elaboration on multi-level nearly-decomposable systems, gives a tentative definition of emergence and discusses how emergent behavior manifests itself in an SoS.