The behavioral effects of lesions of the nucleus basalis magnocellularis (NBM) are reviewed, focusing on the anatomical extent of the lesion, the involvement of neurotransmitter systems and the alterations in memory processes. Most behavioral deficits after NBM lesions can be attributed to damage to the NBM itself, although during spontaneous or pharmacologically induced recovery, other brain structures might play a role. The neurochemical deficit underlying the behavioral impairments is most likely the decrease in cholinergic functioning, since, for example, enhancement of cholinergic functioning is sufficient for behavioral improvement. However, since the lesions are not specific for cholinergic neurons, the extent to which noncholinergic damage causes behavioral deficits is still unclear. Finally, lesions of the NBM impair memory, but affect also other behavioral processes, such as discrimination and habituation. A common process underlying these various impairments could be that of insufficiently focused processing of stimuli.