The traditional methods of characterizing hearing-aid gain as a function of frequency inadequately represent the performance of an aid when it is worn on an individual. Romanow (1942) showed how the body diffraction must be properly accounted for to describe hearing-aid acoustic gain. Hearing-aid audiologists have, for many years, been impressed with the apparent subjective discrepancy between the pressure generated by typical hearing aid in a wearer's ear and the observed response in Standard ANSI S3.3 and IEC measuring schemes. These differences, together with the various legislative pressures for maximizing the utility of the hearing aid for the user, have made the use of additional input to the specification and design of hearing aids essential. The manikin, KEMAR (Knowles Electronics Manikin for Acoustical Research), provides a basis for evaluating many of the parameters which are otherwise difficult or impossible to obtain, but which critically affect the performance of a hearing aid on a typical wearer. Such things as a preferred reference response for a hearing aid that will produce a “subjectively” flat listening stimulus, i.e., a transparent hearing aid, the numerous diffraction effects, the influence of reflection of sound from the torso on a hear mounted hearing aid, the importance of location of the hearing-aid microphone on the head, proper simulation of the ear canal, and a number of other variables can all be studied. These parameters, the directionality of the headworn hearing aid compared to the usual free-field gain measurement and several hearing-aid fitting types, are discussed and typical information is presented. Many of the resulting attributes and benefits from these studies will be more readily implemented in new hearing aids through the use of new generations of microphones and receivers that are available. The evolution of the KEMAR manikin and its acoustic similarity to a median human adult are discussed.