(1) An attempt has been made to discuss the elementary physiological requirements of a mechanical ventilator. (2) The types of machine available are grouped and it is shown that volume constant-pressure variable machmes provide the most satisfactory ventilation. (3) The motive forces used and the machine circuits are described in outline (4) Special features available on (a) operating room machmes and (b) res piratory therapy machmes are described (5) A table is appended, showing certain of the features available on many machines.