The 18th International ACM SIGPLAN Conference on Managed Programming Languages & Runtimes (MPLR'21) was held as a hybrid event at the Alexander-von-Humboldt-Haus in Münster on 29.09.2021 and 30.09.2021. One third of the participants attended in person, while the rest were connected online via Zoom. The complex technical equipment required for this was set up and operated by Matthias and Jonathan Neugebauer. The event was organized by Prof. Herbert Kuchen as General Chair and his team. The program committee was headed by Jeremy Singer from the University of Glasgow. In addition to invited talks by Filipo Pizlo, Apple, USA, and Paul Harvey, Rakuten, Japan, 12 papers were presented that had been successful in peer review. The event concluded with a panel on the challenges and opportunities of managed programming languages.