Lectins are phylogenetically ancient proteins that have specific recognition and binding functions for complex carbohydrates of glycoconjugates, i. e., of glycoproteins, proteoglycans/glycosaminoglycans and glycolipids. This class of proteins mediates important processes of adhesion and communication both inside and outside cells. A large variety of lectins are expressed in the human organism. This article reviews the current knowledge of human lectins with a focus on biochemistry and pathobiochemistry (principles of protein glycosylation and defects of glycosylation as a basis of disease) and cell biology (protein sorting, exocytosis and endocytosis, apoptosis, cell adhesion, cell differentiation, and malignant transformation). The clinical significance of lectin-glycoconjugate interactions is described by example of inflammatory diseases, defects of immune defense, autoimmunity, infectious diseases, and tumor invasion/metastasis. Moreover, therapeutic perspectives of novel drugs that interfere with lectin-carbohydrate interactions are discussed.