Detectors that record charged particles through their ionisation of gases are found in many experiments of nuclear and particle physics. By conversion of the charges created along a track into electrical signals, particle trajectories can be measured with these detectors in large volumes, also inside magnetic fields. The operation principles of gaseous detectors are explained, which include charge generation, gas amplification, operation modes and gas mixtures. Different detector types are described in some detail, starting with ionisation chambers without gas amplification, proceeding to those with gas amplification like spark and streamer chambers, parallel plate arrangements, multi-wire proportional chambers, chambers with microstructured electrodes, drift chambers, and ending with time-projection chambers. The chapter closes with an overview of aging effects in gaseous detectors which cause negative alterations of the detector performance.