Cells in the granular layer of mammalian epidermis contain densely staining bodies called keratohyalin granules. Two types of granules are identified. Larger ones contain phosphorus and consist largely of an unusually histidine rich protein. This protein has a high molecular weight and contains a large fraction of basic aminoacids. However it has a neutral isoelectric point due to extensive phosphorylations. This histidine rich protein undergoes modifications during terminal differentiation, especially when the keratohyalin granules disperse as the granular cells differentiate into the overlying cornified cells. At this point it is dephosphorylated and partially proteolyzed to form lower molecular weight highly basic histidine rich proteins. These cationic molecules aggregate in vitro with keratin intermediate filaments, forming well ordered macrofibrils whose structure resembles the keratin pattern seen in the lower cornified layers. For this reason the name filaggrin is used for these proteins. The high molecular weight precursor is called profilaggrin. Filaggrins are species-distinct products and consist of a number of isoelectric variants. It is established that the filaggrin precursor is composed of tandemly linked multiple copies of filaggrin domains interspersed with short linker peptides. Two functions are proposed for the filaggrin. The first one, based on its interaction with keratin fibres in vitro, is to form the interfilamentous matrix seen in the lower stratum corneum. The second one is to generate a concentrated pool of free aminoacids and derivatives, allowing the stratum corneum to remain hydrated at low environmental humidities. These functions represent sequential rather alternative roles. Epidermal diseases and in vitro cultures illustrate the clear-cut relations between epidermal keratinization and differentiation and the histidine rich proteins pathway.