In this well-written article, Michael Havey explains the two most popular theories that are in vogue among the BPM crowd, which are Pi-Calculus and Petri Nets. The latter served my needs so far in implementing BPM systems I needed. My expertise and understanding of Pi-Calculus is less deep, and I haven't built any systems based on it. There is plenty of rigorous research concerning aplication of Petri Net theory to BPM/Workflow, but on the contrary, there isn't much concerning Pi-C and BPM. It however rules on the hype scale, probably because it is new and less known.