This analysis is centered on the study of cognitive disorders in Alzheimer's disease (AD), mainly for major neuro-psychological functions. We insist on the heterogeneity of the clinical picture peculiarly in the early stages of the illness, even if the deficits of episodic memory and of attentional/executive capacities are the first to deteriorate, preceding impairment in perceptual and language function and potentially having a substantial impact on the patient's capacity to cope independently. An episodic memory deficit is the hallmark of AD, but it must be stressed that this deficit may take different forms and its origin may be traced back to different cognitive mechanisms. One of the most striking aspects of episodic memory impairment in AD is the rapidity of forgetfulness on which screening and diagnostic tests of AD are based. There is some evidence that the episodic memory deficit in AD is one of learning (encoding and storage) of information rather than to a deficit of retrieval. Furthermore, episodic memory performance in AD depends on the integrity of semantic memory abilities, so giving support to a hierarchical model of organization of human memory. Finally, recent results show that an impairment of conscious recollection is responsible for the poor performance of AD patients in recognition memory. Executive deficits appear predominantly in tasks requiring cognitive flexibility and self-monitoring. With the progression of the disease, additional deficits are observed in the verbal concept formation abilities. These findings might be also very useful in the differential diagnosis between AD and the other cortical and subcortical dementias, as well as in the differentiation between AD and fronto-temporal dementia. We consider that studying early stages of the illness is necessary to delineate the diagnostic signs, to validate the new therapeutic experiments, to predict stages of decline. Recent research suggested that onset of AD is commonly preceded by an interim phase known as mild cognitive impairment (MCI). MCI refers to the clinical condition in which persons experience memory loss to a greater extent than one would expect for age, yet they do not meet currently accepted criteria for clinically probable AD. Persons who experience this condition are at increased risk for the development of AD. In MCI, despite the comparable global cognitive functioning, the findings show more impaired retrieval from long-term storage than in NC. The cued recall improves slightly the total recall but the recognition is significantly impaired. Moreover, the data indicate that MCI patients had additional problems with response inhibition, switching and cognitive flexibility. This suggests, that MCI may be identified by using a more detailed procedure for the assessment of cognitive decline than the evaluation of memory alone. As preventive strategies are developed and new cognitive enhancing therapies emerge, these results may also help us to define which domains are expected to improve in MCI populations.