Stochastic activity networks (SANs) are a stochastic generalization of Petri nets that have been defined for the modeling and analysis of distributed real-time systems. SANs were introduced in 1984 for the performance and dependability (performability) modeling of such systems. SAN models are more powerful and flexible than most other stochastic extensions of Petri nets including SPNs and GSPNs. The definition of SANs is based on a unified view of the system in three settings, namely, nondeterministic, probabilistic, and stochastic. The purpose of such a systematic definition is twofold. First, it allows for a better and more formal definition per se. Second, it allows for the use of the model for the analysis of both functional and performance aspects of the system.