Although the gas density balance, devised by Martin, was an extremely effective GC detector, it was not easy to operate and very difficult to construct (particularly as a commercial product). In addition there was a need for greater sensitivity and as a consequence, a number of alternative methods of detection were investigated. The first serious contender to the gas density balance was the katherometer detector which was introduced as a GC detector by Claesson (1). This detector contains a heated filament that is situated in the column eluent. When vapor is present, both the specific heat and the conductivity of the surrounding gas change and consequently its temperature. This causes the resistance of the filament and thus the voltage across it to change which is amplified and fed to a suitable recorder. The details of this detector will be discussed later but, although one of the first to become commercially available, it is still an important detector today and used largely for the detection of permanent gases. The next detector to be developed was the flame thermocouple detector by Scott (2) which, in its original form, employed hydrogen as a carrier gas. The hydrogen was burnt at a small jet in a metal enclosure. A thermocouple was situated over the flame and when solutes were eluted, the temperature of the flame increased and the output from the thermocouple fed to a recorder. This detector was manufactured for a time but was soon developed further by Harley and Pretorious (3) and McWilliams (4) to the flame ionization detector (FID) which is one of the most sensitive and linear GC detectors in use today. Subsequently, a large number of detectors were developed, some of the more effective being the macro and micro argon detectors and the electron 192capture (EC) detector by Lovelock (5–7), the emissivity detector by Grant (8) and the nitrogen/phosphorus detector (NPD) by Karmen and Giuffrida (9). Today almost all GC analyses are carried out using either the FID, the katherometer detector, the EC detector or the NPD. These detectors will be considered in detail, but before doing so, it is necessary to discuss detector specifications and in particular, which specifications are important, how they are defined and how they are measured.