Cerebral asymmetries represent an important principle of the organization of nervous systems. The asymmetries of the avian visual system, with their partly complementary domains in the left and the right hemisphere, offer an excellent window for the lateralized learning and cognitive processes. These behavioural experiments are accompanied by biochemical, synaptic, electrophysiological and neuroanatomical studies which have clarified to some extent the neuronal foundations of this asymmetry. Additionally they show that most, but not all, aspects of visual lateralization depend on a minute asymmetry of prehatch visual stimulation which triggers a cascade of events transforming the embryonic nervous system into lateralized structure and functioning.