Some seeds of flowering plants can differentiate their seed coat epidermis into the specialized cell layer producing a hydrophilic mucilage with several ecological functions, such as seed hydration, protection, spatial fixation, stimulation of metabolic activity and development of seed. Due to the species- and genotype-dependent variabilities in the chemical composition of mucilage, mucilage does not display the same functional properties and its role depends on the respective species and environment. Mucilaginous substances, depending on their composition, exhibit many preventive and curative effects for human and animal health, which has significant potential in the agricultural, food, cosmetic and pharmaceutical industries. This paper summarizes the ecological, biological, and functional properties of mucilaginous plant substances and highlights their significant nutritional potential in terms of the development of functional foods, and nutraceuticals and dietary supplements. A paragraph describing the gene regulation of seed mucilage synthesis is included, and some recommendations for the direction of further research on mucilaginous substances are outlined.