This book constitutes the refereed proceedings of the 12th International Conference on Reachability Problems, RP 2018, held in Marseille, France, in September 2018. The 11 full papers presented were carefully reviewed and selected from 21 submissions. The papers cover topics such as reachability for infinite state systems; rewriting systems; reachability analysis in counter/timed/cellular/communicating automata; Petri nets; computational aspects of semigroups, groups, and rings; reachability in dynamical and hybrid systems; frontiers between decidable and undecidable reachability problems; complexity and decidability aspects; predictability in iterative maps, and new computational paradigms.