An artificial neural network is a computersimulation of a small number of co-operating neurons. A simulation like that can be regarded as a simple model of the functioning of the nervous system. Mental processes can be studied with such models. Recent years have seen an increasing number of publications in which psychopathology has been studied with artificial neural networks. In this article the theoretical concepts of artificial neural networks will be discussed, guided by two types of commonly used networks. It will be shown that the dynamic behaviour of artificial neural networks can be described with the concept 'state space' and that this concept can be used to illuminate the relation between neurobiology and mental processes. The value of artificial neural networks as models is discussed.