This volume gives an introduction to William Ockham’s theory of signs, his ontology, his epistemology, and the interrelations between these fields. Some recent results of analytic philosophy turn out to be useful instruments for the interpretation of Ockham’s thought. Part one contains a presentation of the basic concepts of Ockham’s semantics. It is followed by an investigation of his ontology, including a comparison with modern ontology. Part three deals with the common and different elements in the conceptions of supposition and of quantification. Part four shows some similarities of Ockham’s views on the truth of sentences with those of Davidson. The final part presents Ockham’s epistemology within the discussion of his contemporaries and confronts it with issues raised by Quine and Putnam.