SYNOPSIS. The brain of actinopterygian fishes can be subdivided into five principal parts, rhombencephalon, cerebellum, mesencephalon, diencephalon and telencephalon, each of which contains a number of separate morphological entities: nuclei, areas or zones. Analysis of the origin and termination of the cranial nerves and their components reveals that many of the morphological entities distinguished in the actinopterygian brain can be interpreted in terms of elementary sensory and motor functions. Experimental anatomical and physiological studies on the fiber connections of the entities thus defined have led to a functional interpretation of many other parts of the brain. Thus, the central circuitry related to such sensory functions as hydrodynamic pressoreception, electroreception, vision, gustation and olfaction are well-known, and the same holds true for the motor systems related to feeding and locomotion. However, there are still many regions in the actinopterygian brain the functional significance of which is poorly understood, and it should be emphasized that most of our knowledge on the organization of the brain of this group rests merely on observations in teleosts. One of the most interesting aspects of actinopterygian neurobiology is that the telencephalon in this group develops in a way which differs fundamentally from that found in all other craniates, and that the telencephalon shows a marked progressive differentiation in the series: polypterids—chondrosteans—halecomorphs—teleosts.