This thesis describes the characteristics of a micromachined air-coupled ultrasonic capacitance transducer, using backplates made from chemical etching, ion-beam and laser machining in copper, steel, aluminium and silicon. A series of backplates were produced with micro machined holes on the surface of different depths, diameters and pitch. The resultant characteristics were found to be consistent with previous work. Peak pressure field variations of the capacitance transducers were measured by scanning a detector throughout the radiated field in both air and water. The resulting field compared favourably to a theoretically-generated peak pressure field variation for a plane piston (a FORTRAN program was written which convolved the simulated motion of the membrane with an impulse response for the transducer). Experimental and theoretical peak pressure field variations were measured for an annulus and zoneplate apertures with single and toneburst driving signals. In all cases the experimental field agreed well with the computed theoretical field in air and water. Additionally, with the increase in bandwidth when operated in water, the transducers were used for a pulse-echo C-scan of an artificial defect. Furthermore, the transducers were used to determine the elastic constants in air for glass-epoxy, ceramic and paper card, where the results for glass-epoxy were in good agreement for the values obtained using immersion technique. The transducers were modified by use of a Mica membrane for high temperature use, and were successfully used to monitor the burnout rate of the polymer binder from an injection moulded ceramic.