Portable monitors are classified into three levels (Level II, III, and IV) with decreasing measurements of sleep and respiratory variables. A full overnight sleep study with respiratory measurements and sleep staging (polysomnography) unattended by a sleep technician is Level II, three or more respiratory channels and heart rate generally without sleep staging either attended or unattended is Level III, and one or two channels attended or unattended, usually including oximetry, is Level IV. To date, some Level III portable monitors appear to have sufficient specificity to diagnose the obstructive sleep apnea (OSA) syndrome but are not sufficiently sensitive to exclude OSA. Attended portable monitoring appears to provide better sensitivity and specificity than unattended portable monitoring and is an option for diagnosis of OSA. The role of portable monitoring is evolving but at this time cannot substitute for attended polysomnography as a standalone approach. The exact place of portable monitoring and the cost-benefit depends on local circumstances and cannot be generalized at this time.