In recent decades, the importance and extent of control of the leak tightness of devices, systems, components, storage tanks, etc. has greatly increased, not only in the vacuum industry and in industries where vacuum traditionally plays an important role, but also and especially in industrial processes not directly related to vacuum activities. The ever growing number of economically or environmentally demanded applications of tightness control in industry and science has made leak detection a more or less independent discipline. With respect to the Dutch version of this book from 2000, the chapter is totally revised and contains significant innovations. Thus, in helium leak detection the main flow principle has come off worst with respect to the counterflow principle. This is mainly because of the more robust system, less chance of contamination of the mass spectrometer and the user-friendliness. Leak detectors according to the main flow principle are still extensively used in the laboratory environment, but are no longer commercially available. Furthermore, the palette of inside-out sniffing systems has greatly expanded. In addition to the long-standing helium and halogen sniffers, the hydrogen leak detector, quartz window sensor and several multigas sniffing systems, such as the infrared leak detector, are now on the market. Finally, two newly developed inside-out detection methods are added, the so-called ‘atmosphere method’ and ‘bombing’. All detection methods are evaluated for their sensitivity, response time, ease of use and overall employability.