Pulsatile drug delivery systems are developed to deliver drug according to circadian behavior of diseases.The product follow a sigmoidal drug release profile characterized by a time period of no release (lag time) followed by a rapid and complete drug release.Thus drug can be delivered at right time, in right amount and at right site of action by use of such approach.The potential benefits of chronotherapeutics have been investigated and established for number of diseases like asthma, arthritis, cancer, diabetes, epilepsy, hypertension, ulcer, hypercholesterolemia etc. Various capsular, osmotic, single and multiple unit systems that are modulated by soluble or erodible polymer coatings, rupturable membranes are available in market.These systems are beneficial for diseases showing chronopharmacological behavior where night time dosing is required or for the drugs having high first pass effect or having site specific absorption in GIT, or for drugs with high risk of toxicity or tolerance.These systems also improve patient compliance by decreasing dosing frequency. INTRODUCTION:Today, a vast amount of literature reports that biological processes are not constant but vary according to time.Although much of drug delivery research has focused on constant drug release rate due to limitations of delivering drug according to disease rhythmicity, clinical studies show that magnitude of rhythmic differences can be to a great extent and a strong determinant of when during 24 hour most morbid and mortal event will occur.For many drugs constant release system is not suitable.Drugs not suitable for constant release are used in disease condition that exhibit rhythmic variation within a circadian cycle.For, drugs with decrease bioavailability due to first pass metabolism, gradual release of drug from constant release systems can result in greater degradation.