1. Introduction.- 2. The Petri Box Calculus.- 3. Syntax and Operational Semantics.- 4. Petri Net Semantics.- 5. Adding Recursion.- 6. S-invariants.- 7. The Box Algebra.- 8. PBC and Other Process Algebras.- 9. A Concurrent Programming Language.- 10. Conclusion.- Appendix: Solutions of Selected Exercises.- References.