Several important GC detectors are based on the principle of ionization of the solute and measurement of the ion current generated. Of these, the Flame lonization Detector (FID) is the most important and has the best characteristics for quantitative gas chromatography. The FID has a linear range of over 107, is applicable to a wide range of solutes with high sensitivity, and is relatively insensitive to operating variables. This paper describes the principles of operation of the FID and critically examines the effect of important operating parameters such as burner jet I.D., collector electrode gap, and hydrogen flow rate. Four performance factors (sensitivity, response factor, response index, and calibration range) are defined and explained. These are useful for comparative evaluations of FID's of different design. Other ionization detectors are compared and contrasted with respect to these four factors. An electrometer is required for measurement of ionization detector currents. Design criteria for GC electrometers are described, including compatibility with various detectors, detector polarization, input and output signal level controls, sensitivity, stability, and linear range. The design of a tube solid-state hybrid GC electrometer is described with regard to these criteria. Performance specifications of the hybrid electrometer are contrasted with 3 other types—a Field Effect Transistor (FET), a Varactor Bridge, and a Vibrating Reed type. Examples will be given which demonstrate the performance capabilities and limitations of an FID and electrometer system.