An Essay on Sabotage and Obstruction Johan van Benthem Abstract: We investigate the complexity jump which occurs when an algorithm of known complexity is turned into a game by adding an opposing player. Our specific examples are reachability and circuit algorithms, which now have a Demon trying to prevent successful completion. We get a PSPACE upper bound by translating the problems into first-order model checking tasks on finite models. Finally, we consider general jumps in coalitional powers when we add players to a game. Keywords: algorithm; game; sabotage; complexity