We develop techniques for learning the meanings of unknown words in context. Working within a compositional semantics framework, we write down equations in which a sentence' s meaning is some combination function of the meaning of its words. When one of the words is unknown, we ask for a paraphrase of the sentence. We then compute the meaning of the unknown word by inverting parts of the semantic combination function. This technique can be used to learn word-concept mappings, decomposed meanings, and mappings between syntactic and semantic roles. It works for all parts of speech.