Major difficulties in language processing are caused by the fact that many words are ambiguous, i.e. they have different meanings in different contexts, but are written (or pronounced) in the same way. While syntactic ambiguities have already been addressed in the previous chapter, now the focus is set on the semantic dimension of this problem. In this chapter, the problem of word sense ambiguity is discussed in detail. A Structure Discovery process is set up, which is used as a feature to successfully improve a supervised word sense disambiguation (WSD) system. On this basis, a high-precision system for automatically providing lexical substitutions is constructed.