The types of testing devices are categorized according to the number and type of physiologic parameters that are monitored. The categorization currently used by CMS is listed in Table 1.11 This differs slightly from the original categorization presented in the ASDA [American Sleep Disorders Association] Standards of Practice: Portable Recording for the Assessment of Obstructive Sleep Apnea in 1994.12 The “gold standard” study is a type I test, which is an attended, in-laboratory polysomnography. This test includes a minimum of the following seven parameters: EEG, electrooculogram (EOG), chin electromyogram (EMG), oxygen saturation, airflow, respiratory effort, and ECG. “Attended” is defined as “a technologist or qualified health-care professional is physically present (ie, sufficient proximity such that the qualified health-care professional can physically respond to emergencies, to other appropriate patient needs or to technical problems at the bedside) throughout the recording session.”13 Type II testing is defined as meeting the criteria of a type I test but in an unattended setting. Unattended is defined as “a technologist or qualified health-care professional is not physically present with the patient during the recording session.”13 Type III testing includes monitoring of a minimum of the following four parameters: ECG/heart rate (HR), oxygen saturation, and two channels of respiratory movement or, alternatively, one channel of respiratory movement and one channel of airflow. A type IV test includes measurement of three physiologic parameters. This definition differs from the original definition of a type IV test, which included single- or dual-channel, continuous, bioparameter recording, but is more clinically applicable to the devices used today. Recently, a more descriptive classification system for the devices used in out-of-center testing has been proposed, but it has not yet been adopted by payers.14 One should be familiar with the strength and weaknesses of the various technologies, but it is beyond the scope of this document to review this.