The logic of relevance was first introduced into the subject-matter of logical investigations by Ackermann and Church, and has since been explored in great detail by Anderson, Belnap and their associates. Diaz's book is, after Anderson's and Belnap's Entailment, the most important exploration of this theme. It puts forward new and original results concerning relevance-systems and at the same time offers the first attempt to criticise and - by means of a radical employment of Kripke-style semantics - to overcome the ad hoc character of criteria of relevance. Diaz's investigation is an important contribution to and to the philosophy of logic.