The paper consists of two parts. In the first part, the rationale and considerations leading to the development of an explosives vapour detector suitable for screening suspect aircraft for hidden bombs is discussed. The detector is based on GC/ECD and consists of two components, the main analysing unit and a detachable sampling probe. A number of prototype detectors was fabricated at the National Research Council of Canada (NRC), and, after laboratory and field trials, the design became the basis of a commercially produced model.