This is the first of two papers reviewing the SISI test. Following a discussion of the history of clinical intensity discrimination tests, a large body of data on the SISI test is reviewed with special attention to its procedure, as well as its validity and reliability. The following conclusions are drawn: (1) Sufficient practice should be given prior to the onset of data collection. (2) The number of presentations can safely be reduced to 10 if the patient either responds consistently or does not respond at all. (3) SISI possesses good validity and reliability. Used in conjunction with other tests, it gives valuable diagnostic information.