Welcome to MPLR 2024, the 21st ACM SIGPLAN International Conference on Managed Programming Languages and Runtimes, held in Vienna, Austria on Thursday, September 19, 2024, co-located with ISSTA/ECOOP 2024. MPLR is a successor to the conference series on Managed Languages and Runtimes (ManLang, 2017 and 2018) which in turn is a successor to the conference series on Principles and Practice of Programming in Java (PPPJ, 2002 through 2016). MPLR is a premier forum for presenting and discussing novel results in all aspects of managed programming languages and runtime systems, which serve as building blocks for some of the most important computing systems around, ranging from small-scale (embedded and real-time systems) to large-scale (cloud-computing and big-data platforms) and anything in between (mobile, IoT, and wearable applications).

Our program includes a keynote talk by Ben L. Titzer, 7 full papers, and 4 shorter work-in-progress papers. These were selected from 11 full paper submissions and 1 work-in-progress submission (3 full paper submissions were selected as work-in-progress papers). All papers received at least 3 reviews from the program committee, listed below. Full papers were evaluated based on relevance, novelty, technical rigor, and contribution to the state of the art. Work-in-progress papers were reviewed based more on novelty and probable interest to the community. The program also includes 2 posters of which one is a poster version of an accepted paper. The posters received at least 2 light reviews each. Reviewing was double blind, the second time for MPLR. The review process was all online with consensus reached on each submission. We used the HotCRP management system and Conference Publishing handled the proceedings. We are grateful to the program committee for their diligence in reviewing, the quality of the reviews, and productive nature of the discussion process.

We hope that readers will find these proceedings interesting and that conference attendees will enjoy MPLR 2024, be they physically or virtually present!