Since the early nineteenth century, companies manufactured devices specially designed to assist the hard of hearing in conversation. In Britain these were available commercially from high street vendors of prosthetics, or for purchase by mail order; in neither context were they primarily marketed or purchased as medical products. Many such mass manufactured devices were patented to secure protection against plagiarists in this lucrative market, or to signal their presumptive efficacy. By contrast, clinical interventions rarely provided therapeutic relief, sometimes even exacerbated hearing loss. So up to the early twentieth century, medical involvement was customarily limited to advice on reputable high street vendors of hearing aids. In treating victims of brutal combat conditions in Europe’s Great War, however, it became clear that hearing loss could be both a physiological and psychological phenomenon. Certain kinds of disability assistance were recognized by governments as appropriate to hearing loss newly valorised as a war injury deserving compensation. Thus in the interwar period, the medical profession took a greater interest, staking a claim in this territory. Some manufacturers took advantage of this and solicited medical inspection and approval of their products to differentiate themselves from downmarket opportunistic mail order products. This was especially important for assuring the safety and efficacy of new kinds of electronic amplification derived from telephony and radio. Such devices proved to be especially important supporting the many victims of hearing loss created in Second World War combat conditions. Indeed the sheer scale of those conditions precipitated the rise of the new medical profession of audiology whose specialist prerogative it was to diagnose the particularities of each human subject’s condition and to prescribe a tailored technological solution. This harmonious symbiosis of technical expertise and medical authority in handling hearing loss began to break down with the arrival of the National Health Service in 1948. In that year, free hearing aids became standardly available for a large low-income constituency of the population. These "Medresco" models were produced by the Post Office telecommunications section in collaboration with the Medical Research Council: this appalled and alienated the well-established community of private commerce which was thereby completely excluded from state hearing aid provision. The uncomfortable rift took a rather different form in the USA. There hearing aid manufacturers kept much stronger control over access to hearing aids within private healthcare provision. Indeed it was not until the 1970s that US audiologists were permitted to prescribe hearing aids at all. Overall then we see that the story of hearing aids sheds light on the complex contingencies through which the medical professions and commercial industry delineated boundaries for their prerogatives in healthcare.