Self-powered detectors are being used in an increasing number of heavy and light water reactors in both safety and flux mapping systems. The types available can be divided into neutron and gamma sensitive devices, with the neutron detectors further subdivided into those with a delayed response (the most common examples are Rh and V devices) and those with an instantaneous response (for example Co). The gamma sensitive devices, of which Pt is the most common example, all have instantaneous response. The properties of these detectors are related to their operating mechanisms and the relationship between them will be reviewed in the paper. These differences result in a series of detectors each with their own favorable and unfavorable characteristics. For example, Rh detectors produce the highest current of those considered but suffer from a delayed response (95% response in about 150 sec), and rapid burnup. Co detectors produce an instantaneous response, and burn up more slowly, but their current is about 1/60 that of Rh and they suffer from the buildup of a long lived background from neutron activation. Finally Pt detectors also produce an instantaneous response and have almost no burnup but their current is approximately 1/10 that of Rh and their response involves a mixture of neutron and gamma induced currents. At Westinghouse, we have been studying these detector characteristics by means of a Monte Carlo program written for that purpose and some of the insight gained from these calculations will be described in the paper.