The term `spatial hemineglect' refers collectively to disorders of spatial cognition, which concern specific sectors of space with reference to a given coordinate system. Patients with cerebral lesions involving the posterior-inferior parietal and the premotor cortex, most often in the right hemisphere, sometimes fail to explore the extra-personal and personal sectors of space contralateral to the side of the lesion, are not aware of stimuli presented in these portions of space, or of contralateral body parts and their disordered function. In addition to these negative signs patients may also show positive pathological manifestations, such as avoidance or withdrawal from the contralateral side of space, and delusional views concerning contralateral body parts. The many varieties of this disorder can occur in dissociated forms, suggesting a multifaceted organization of the internal representation of space, of spatial attention, and of their neural correlates. Many manifestations of hemineglect are modulated in a similar fashion by specific sensory stimulation that also affects visuo-motor processes in normal subjects. This ongoing sensory modulation might update the internal representations of space in a continuously changing environment, and contribute to the formation, around the vertical orientation of gravity, of our subjective unitary experience of space.