An evaluation of monitoring techniques that range from "gadgets" to potential lifesavers, but are, in general, simply new ways of displaying old information much of which the surgeon can detect or deduce by more direct means. Their value has been in establishing physiologic principles that, once established, make the electronic equipment less important. Advances may be expected in three areas: (1) in theoretical analysis, where computational techniques enable the surgeon to make multiple correlations to identify significant variables, and to use continuous "trend analysis"; (2) in technical hardware, such as needle transducers and improved noninvasive techniques for recording arterial blood pressure; and (3) in system design, where the monitoring device will not only detect abnormalities but will initiate corrective therapy through preprogrammed logic. As new and beneficial monitoring aids are provided, the surgeon must separate in his own mind the equipment used for monitoring from the physiologic principles that regulate patient condition during surgery. 11 pp.