Systems development methods have become more complex, concurrently with many of today’s systems. UML has been criticized for its complexity. Using Rossi and Brinkkemper’s (1996) complexity metrics, Siau and Cao, (2001) completed a complexity analysis of UML and other modeling techniques, finding that UML is more complex than other techniques. Siau, Erickson and Lee (2002) proposed that Rossi and Brinkkemper’s metrics present the theoretical maximum complexity, as opposed to a practitioner-based complexity, that must be less than the theoretical maximum. Erickson and Siau (2004) proposed that a subset of UML (a kernel) composed of the most commonly used constructs, could be equated with the complexity that practitioners face when using UML. The current research extends Erickson and Siau’s (2004) work by analyzing UML complexity in the specific genre of real time systems. A Delphi study was conducted using expert practitioners, in an attempt to identify a use-based real-time UML kernel.