Obstructive sleep apnea syndrome (OSAS) is characterized by repeated oropharyngeal occlusions occurring during sleep. The prevalence of moderate OSAS (with an apnea-hypopnea index = or >15/h) is 9% and 4% in male and female, respectively. It is associated with an abnormally high frequency of cardiovascular disease (hypertension, stroke, coronary heart disease) and excessive daytime sleepiness responsible for an increased frequency of work and road accidents. Because the treatment of OSAS provides many benefits to patients and society, it is very important to obtain an early diagnosis. The diagnosis of OSAS is based on the combination of characteristic clinical features plus compatible findings on instrumental tests in which multiple physiologic signals are monitored simultaneously during a night of sleep. A full night polysomnography, conducted by a technologist in a sleep laboratory, is the gold standard for the diagnosis of suspected OSAS, but the capacity for performing polysomnography is limited. On the basis of the high incidence and prevalence of OSAS, of the limited number of sleep laboratories, long waiting times and high costs recommendations have been formulated for the use of unattended portable systems in the assessment of OSAS. The main clinical aspects of of OSAS, the diagnostic approach with full night polysomnography and unattended portable systems, the differential diagnosis and some examples of cardiorespiratory portable monitoring are presented in this paper.