Obstructive sleep apnea (OSA) is a condition of repetitive upper airway collapse, which occurs during sleep. Recent literature has emphasized the role of OSA in contributing to glucose intolerance, dyslipidemia, and hypertension. OSA is associated with the development of cardiovascular disease, although definitive data are sparse with regard to the prevention of cardiovascular disease and CPAP therapy. CPAP provides effective treatment for OSA, but patient adherence remains challenging. Aside from daytime symptom improvement, it is difficult to monitor the adequacy of treatment response. Thus, the search for a biomarker becomes critical. The discovery of an ideal biomarker for OSA has the potential to provide information related to diagnosis, severity, prognosis, and response to treatment. In addition, because large-scale randomized controlled trials are both ethically and logistically challenging in assessing hard cardiovascular outcomes, certain biomarkers may be reasonable surrogate outcome measures. This article reviews the literature related to potential biomarkers of OSA with the recognition that an ideal biomarker does not exist at this time.