Excerpted from the "ADA Title III Regulation 28 CFR Part 36" http://www.ada.gov/reg3a.html
Sec.36.303 Auxiliary aids and services.
(a) General. A public accommodation shall take those steps that may be necessary to ensure that no individual with a disability is excluded, denied services, segregated or otherwise treated differently than other individuals because of the absence of auxiliary aids and services, unless the public accommodation can demonstrate that taking those steps would fundamentally alter the nature of the goods, services, facilities, privileges, advantages, or accommodations being offered or would result in an undue burden, i.e., significant difficulty or expense.
(b) Examples. The term "auxiliary aids and services'' includes --
(1) Qualified interpreters, notetakers, computer-aided transcription services, written materials, telephone handset amplifiers, assistive listening devices, assistive listening systems, telephones compatible with hearing aids, closed caption decoders, open and closed captioning, telecommunications devices for deaf persons (TDD's), videotext displays, or other effective methods of making aurally delivered materials available to individuals with hearing impairments;
(2) Qualified readers, taped texts, audio recordings, Brailled materials, large print materials, or other effective methods of making visually delivered materials available to individuals with visual impairments;
(3) Acquisition or modification of equipment or devices; and
(4) Other similar services and actions.
(c) Effective communication. A public accommodation shall furnish appropriate auxiliary aids and services where necessary to ensure effective communication with individuals with disabilities.
(d) Telecommunication devices for the deaf (TDD's).
(1) A public accommodation that offers a customer, client, patient, or participant the opportunity to make outgoing telephone calls on more than an incidental convenience basis shall make available, upon request, a TDD for the use of an individual who has impaired hearing or a communication disorder.
(2) This part does not require a public accommodation to use a TDD for receiving or making telephone calls incident to its operations.
(e) Closed caption decoders. Places of lodging that provide televisions in five or more guest rooms and hospitals that provide televisions for patient use shall provide, upon request, a means for decoding captions for use by an individual with impaired hearing.
(f) Alternatives. If provision of a particular auxiliary aid or service by a public accommodation would result in a fundamental alteration in the nature of the goods, services, facilities, privileges, advantages, or accommodations being offered or in an undue burden, i.e., significant difficulty or expense, the public accommodation shall provide an alternative auxiliary aid or service, if one exists, that would not result in an alteration or such burden but would nevertheless ensure that, to the maximum extent possible, individuals with disabilities receive the goods, services, facilities, privileges, advantages, or accommodations offered by the public accommodation.