The spark chamber is introduced in a historical review following the evolution of the instrument from the d.c. spark counter, and its present position compared with that of other instruments in the same class. A largely qualitative treatment describes the theory of spark formation, gap efficiency, and accuracy of track location. Practical matters relating to design and construction and the operation of chambers are discussed, and brief descriptions given of those employing special electrode configurations. Reference is made to the development of electronic data transfer from spark chambers, and the future possibilities of such methods.