[en] The main parameters are defined and described of an electrometer, including input resistance, input quiescent current, current noise equivalent, voltage and current stability, minimum input capacity, response time, time constant, range, accuracy, linearity, a-c component suppression, and zero drift. The limiting factors in measurement mainly include temperature noise, insulator quality, radioactivity background, electrostatic and electromagnetic interference, contact potential difference, and resistor stability. Electrometers are classified into three basic groups, viz., electrostatic electrometers, d-c amplifier-based electrometers (electron tube electrometers and FET electrometers), electrometers with modulation of measured signal (electrometers using vibration capacitors, electrometers with varactors). Diagrams and specifications are presented for selected electrometers. (J.B.)