This lecture covers the principles and operation modes of gaseous ionisation detectors, including ionisation chambers and proportional counters. It explains the detection regions, signal processing modes, and spectrometry measurements. The lecture also discusses the interest, applications, and properties of ionisation chambers and proportional counters, as well as Geiger-Müller counters. It concludes with a comparison of gaseous detectors and their typical uses in radiation detection and dosimetry.