Therapeutic trials conducted in Alzheimer's disease have benefited from the standardization of diagnostic criteria based on internationally recognized scales (DSM III-R, NINCDS-ADRDA) which ensure more valid inclusions. Well specified exclusion criteria are also of the utmost importance, in particular depression, vascular dementia and concomitant psychotropic drugs. Cognitive and/or functional scales allow an appreciation of the severity of the disease. Due to the heterogeneity of Alzheimer's disease stratification methods on identified prognostic factors i.e. aphasia, extrapyramidal symptoms should be performed. Selection of responders during an enrichment phase has still to be discussed. Multicentric studies become imperative because of the large number of patients required and the difficulties in selecting the adequate patients. These raise the issues of investigators' experience, coordination and between center variability.