It is a survey of some past work on mechanical teaching aids. In this paper Porter makes an important distinction among devices which provide Stimulus functions only (such as usual forms of audio-visual aids), those which provide response apparatus only (such as Pressey's punchboard), and those which provide for both Stimulus and response functions. As examples of the last class, Porter cites Pressey's early self-testing and teaching devices and the first machines developed in Skinner's laboratory, pointing out the similarities and differences in the two approaches. Two important innovations made by Skinner are brought out here - the use of constructed answers rather than multiple-choice responding and the carefully organized programming of subject matter. Porter evaluates four previous experimental studies reported in Part II of this volume.