This book constitutes the proceedings of the 8th International Conference on Rigorous State-Based Methods, ABZ 2021, which was planned to take place in Ulm, Germany, during June 6-11, 2021. The conference changed to an online format due to the COVID-19 pandemic. The 6 full and 8 short papers included in this volume were carefully reviewed and selected from 18 submissions. The proceedings also include 3 PhD symposium contributions. They deal with state-based and machine-based formal methods, mainly Abstract State Machines (ASM), Alloy, B, TLA+, VDM, and Z.