Detection of a signal from explosives at the level of a few parts in a trillion presents a formidable challenge for analysts who use detectors which rely on the chemical signatures of the material. Whether in vapor form in air, or in solid form in a matrix of ambient debris, the target chemical compounds may be present in only minute quantities compared to a large mass or volume of innocuous material. To be practicable in routine use by security personnel the false alarm rate from any detector must be very low, and the speed of analysis must be rapid to maintain a high throughput of samples. Thus, the main signal detection criteria can be defined, based on the following requirements: . very high sensitivity to specific compounds of interest, . high specificity to these compounds in order to maintain a very low false alarm rate, and . rapid data acquisition and data processing times to provide instantaneous (or real-time) detection. This paper outlines a detection process based on a tandem mass spectrometer coupled with an ionization source that operates at atmospheric pressure (APITMSTMS). This instrument has the intrinsic high sensitivity of mass spectrometry, particularly with respect to compounds such as explosives, and achieves the required high specificity and throughput by a series of rapid, but effective, filtering steps. While sensitivity is important, selectivity or the ability to discriminate between analytes of interest and the background signal is the most important factor in obtaining very low detection limits. Background signals are comprised both of electronic and chemical noise, with the sample matrix chemical interference being the primary potential source of background. Chemical and ion optical filtering of ionized vapor samples is used to select only those ions which have the correct massto-charge ratio corresponding to the target compounds. These selected ionized molecules are then introduced into a region of the MS/MS where the ions are collisionally fragmented to produce fragment ion spectra (referred to as daughter ion spectra) which are related to the structures of the initial target molecules. By identifying and matching the daughter spectral peaks with known target compound spectra, explosive compounds can be specifically detected with low detection limits.