The underlying theory for modistic grammar is Aristotelian metaphysics, especially the theory of the categories. This paper deals with the semantics of the categories as viewed by one of the most important medieval commentators, Albert the Great. Albert’s comments on the categories of substance, quantity, and quality are considered, and the relation of these to the other seven categories is also discussed. The categories are modes of predicating with respect to what the predicates signify. Substance words are predicated univocally while terms signifying accidents are predicated paronymously. Secondary substance signifies quale quid; hence common nouns are names of kinds. Primary substance signifies hoc aliquid, the concrete individual; for this reason proper nouns are grammatical subjects but not genuine predicates. Quantity and quality are simple and absolute modes or predication. They are intrinsic accidents because they are based on the matter-form composition of substance. An interesting aspect of the Aristotelian conception of quantity is the view that speech is a discrete quantity. Quality words signify quale rather than quale quid; hence the distinction between sortal and characterizing predicates is a basic idea of Aristotelian ontology. As for the other categories, relation is a simple but not an absolute mode of predication. The other six categories all express some mode of relatedness and thus are regarded as extrinsic principles of being.