  Bowen showed that a continuous expansive map with specification has a unique
measure of maximal entropy. We show that the conclusion remains true under
weaker non-uniform versions of these hypotheses. To this end, we introduce the
notions of obstructions to expansivity and specification, and show that if the
entropy of such obstructions is smaller than the topological entropy of the
map, then there is a unique measure of maximal entropy.
