This lecture covers the principles and operation of gaseous ionisation detectors, including ionisation chambers, proportional counters, and Geiger-Müller counters. It explains the detection regions, operation modes, and spectrometry measurements. The instructor discusses the interest, applications, and properties of ionisation chambers, proportional counters, and Geiger-Müller counters, highlighting their advantages and disadvantages. The lecture also explores the formation of avalanches, choice of detector geometry, and variants/upgrades in detector technology.