The mucosal maltase enzymes are characterized by an activity that produces glucose from linear glucose polymers, assayed with the disaccharide maltose. The related enzyme isomaltase produces glucose from branched glucose polymers, assayed with palatinose. Maltase and isomaltase activities are part of the 4 disaccharidases assayed from clinical duodenal biopsy homogenates. The reported maltase activities are more difficult to interpret than lactase or sucrase activities because both the sucrase-isomaltase and maltase-glucoamylase proteins have overlapping maltase activities. The early work of Dahlqvist identified 4 maltase activities from human small intestinal mucosa. On one peptide, sucrase (maltase Ib) and isomaltase (maltase Ia) activities shared maltase activities but identified the enzymes as sucrase-isomaltase. On the other peptide, no distinguishing characteristics of the 2 maltase activities (maltases II and III) were detected and the activities identified as maltase-glucoamylase. The nutritional/clinical importance of small intestinal maltase and isomaltase activities are due to their crucial role in the digestion of food starches to absorbable free glucose. This review focuses on the interpretation of biopsy maltase activities in the context of reported lactase, sucrase, maltase, and palatinase biopsy assay activity patterns. We present a classification of mucosal maltase deficiencies and novel primary maltase deficiency (Ib, II, III) and provide a clarification of the role of maltase activity assayed from clinically obtained duodenal biopsies, as a path toward future clinical and molecular genomic investigations.