This communication offers an attempt to classify the different substrates used for hydroponics. This classifications is based on different criterions according to the properties of the materials: a. origin of the material; b. physical properties; c. reaction on the nutrient solution (chemical and physico-chemical properties of the substrate) . The different types of substrate are discussed in accordance with their classification and the data mentioned in the literature. Special reference will be made to the volcanic materials used in the Canary Islands and to the effect of being used as substrate for hydroponics during many years.