The nasal cycle is classically defined as rhythmic, alternating side to side fluctuation in nasal airflow. It is known to be regulated by autonomic nervous system probably from the centres located in brainstem. Its classical form requires identical periods and equal amplitude of patency changes, with the total airflow remaining unchanged. This pattern was previously shown to occur in 75% healthy adults. In more recent studies is has been shown that spontaneous changes in nasal patency are in most cases of more irregular pattern. This article describes methods of detection, classification, current views on the role of the cycle.