Almost 30 years ago, the research on process calculi gained a lot of momentum with the invention of ACP, CCS and CSP. Later on, but also already 20 years ago, researchers started to consider so-called mobile variants of process calculi, in which communication channels were themselves treated as the exchanged data. The original Pi us arose out of a reformulation and extension of CCS. In turn, it boosted the invention and study of a whole zoo of further process calculi.