Ultrasonic sensors in air are intelligent sensors, which use wave-propagation phenomena in air to measure physical or chemical variables. With accordance to the influence principle, two main types of ultrasonic sensors can be differentiated. Propagation-path sensors decode changes on propagation to obtain a fast measurement of temperature, pressure variations or gas concentration. Most important are ultrasonic flow sensor e.g. for engine intake air or for air/gas measurement. Distance sensors detect echoes from objects and evaluate their propagation time and amplitude. Examples are distance meters and presence detectors. By intelligent algorithms based on signal theory models or heuristic approaches (synthetic aperture, pulse holography, fuzzy or neural network etc.) the resolution and detection range were increased, the radial and lateral resolution significantly enhanced and objects recognized. With piezoelectric PVDF-foils efficient and versatile transducers arrays were realized. Robust transducers for operation in adverse environment (dirt, moisture, abrasion) were developed using rugged matching layers or a special composite-transducer technology