noses, each sensor is sensitive to all volatile molecules but each in their specific way. However, in bio-electronic noses, receptor proteins which respond to specific odor molecules are used. Most electronic noses use chemical sensor arrays that react to volatile compounds on contact: the adsorption of volatile compounds on the sensor surface causes a physical change of the sensor. A specific response is recorded by the electronic interface transforming the signal into a digital value. Recorded data are then computed based on statistical models. Bio-electronic noses use olfactory receptors – proteins cloned from biological organisms, e.g. humans, that bind to specific odor molecules. One group has developed a bio-electronic nose that mimics the signaling systems used by the human nose to perceive odors at a very high sensitivity: femtomolar concentrations. The more commonly used sensors for electronic noses include metal–oxide–semiconductor (MOS) devices – metal–oxide–semiconductor sensors contain a metal oxide coating with an electrical resistance that changes in the presence of a target gas. The presence of the target gas can be inferred by measuring the change in the resistance of the metal oxide layer over time. conducting polymers – organic polymers that conduct electricity. polymer composites – similar in use to conducting polymers but formulated of non-conducting polymers with the addition of conducting material such as carbon black. quartz crystal microbalance (QCM) – a way of measuring mass per unit area by measuring the change in frequency of a quartz crystal resonator. This can be stored in a database and used for future reference. surface acoustic wave (SAW) – a class of microelectromechanical systems (MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon. Mass spectrometers can be miniaturised to form general purpose gas analysis device. Some devices combine multiple sensor types in a single device, for example polymer coated QCMs. The independent information leads to vastly more sensitive and efficient devices. Studies of airflow around canine noses, and tests on lifesize models have indicated that a cyclic 'sniffing action' similar to that of a real dog is beneficial in terms of improved range and speed of response In recent years, other types of electronic noses have been developed that utilize mass spectrometry or ultra-fast gas chromatography as a detection system. The computing system works to combine the responses of all of the sensors, which represents the input for the data treatment. This part of