We introduce and investigate input-driven stack automata, which are a generalization of input-driven pushdown automata that recently became popular under the name visibly pushdown automata. Basically, the idea is that the input letters uniquely determine the operations on the pushdown store. This can nicely be generalized to stack automata by further types of input letters which are responsible for moving the stack pointer up or down. While visibly pushdown languages share many desirable properties with regular languages, input-driven stack automata languages do not necessarily so. We prove that deterministic and nondeterministic input-driven stack automata have different computational power, which shows in passing that one cannot construct a deterministic input-driven stack automaton from a nondeterministic one. We study the computational capacity of these devices. Moreover, it is shown that the membership problem for nondeterministic input-driven stack automata languages is NP-complete.