We study the complexity of several recently proposed methods for updating or revising propositional knowledge bases. In particular, we derive complexity results for the following problem: given a knowledge base T, an update p, and a formula q, decide whether q is derivable from Top, the updated (or revised) knowledge base. This problem amounts to evaluating the counterfactual p > q over T. Besides the general case, also subcases are considered, in particular where T is a conjunction of Horn clauses, or where the size of p is bounded by a constant.