Students who do not maintain continuous enrollment are required to re-apply for admission (or apply for re-admission) before returning to the program.
Continuous enrollment means not going more than two quarters, including Summer Quarter, without registering for courses during any one year period. Since no graduate courses are offered during Summer Quarter, this really means that one cannot skip any two consecutive quarters (Fall, Winter; Winter, Spring; or Spring, Fall) without having to reapply.
Re-applying means at least having to fill out the forms and pay the fee. If one is gone too long, it may also be necessary to obtain a new set of transcripts, etc.
Students may be granted a leave of absence when it is necessary for reasons of health, employment, etc., and when petitioned for in advance. Such leaves may not be granted retroactively.