Various devices are described which record physical phenomena and which respond by transmitting signalsviainformation processing units to actuators. For example, a thermistor senses temperature and transmits an electrical signal to an integrated circuit which may in turn actuate a furnace control. This review emphasizes the structure-property relationships of the sensor materials. An ionic conductor, such as stabilized zirconia, can be used to detect oxygen in air/fuel mixtures; metal oxides of large surface area can monitor humidity by surface conduction; and semiconducting transition-metal oxides can measure pH. The mechanism of vanadium dioxide critical temperature thermistors is described; as well as the operation of thermistors of negative and positive temperature coefficients (NTC and PTC) made from, for example, doped nickel oxide and barium titanate respectively. Composite sensors of two or more phases have properties which are the product of those of the individual components and have wide application. For example, combining the Hall effect with electrical conductivity will give magnetoresistance, a property which can be used in a composite magnetic field sensor. Two non-magnetoelectric materials can be cleverly combined to form a magnetoelectric composite. A composite material will exhibit only those symmetry elements that are common to its constituent phases and to their geometrical arrangement. The microstructures are given of composite thermistors and humidity sensors. A variety of integrated sensors can be built into single-crystal silicon chips to detect gases and to act as accelerometers, their fabrication involving micromachining and both isotropic and anisotropic etching.