Reachability Problems : 5th International Workshop RP 2011 Genoa Italy September 28 30 2011. Proceedings This book constitutes the refereed proceedings of the 5th International Workshop on Reachability Problems RP 2011 held in Genoa Italy in September 2011. The 16 papers presented together with 4 invited talks were carefully reviewed and selected from 24 submissions. The workshop deals with reachability problems that appear in algebraic structures computational models hybrid systems logic and verification. Reachability is a fundamental problem that appears in several different contexts: finite and infinite state concurrent systems computational models like cellular automata and Petri nets decision procedures for classical modal and temporal logic program analysis discrete and continuous systems time critical systems and open systems modelled as games.