We present three approaches to word sense disambiguation that use Wikipedia as a source of sense annotations. Starting from a basic monolingual approach, we develop two multilingual systems: one that uses a machine translation system to create multilingual features, and one where multilingual features are extracted primarily through the interlingual links available in Wikipedia. Experiments on four languages confirm that the Wikipedia sense annotations are reliable and can be used to construct accurate monolingual sense classifiers. The experiments also show that the multilingual systems obtain on average a substantial relative error reduction when compared to the monolingual systems.