An important concept in retailing is impulse buying. This phenomenon has been studied in consumer behavior research as well as in psychology and economics since 1950s. Impulse purchase is generally unplanned purchases where purchase decisions are made on the spot. By some estimates 50 percent of all consumers' spending is on impulse. With e-commerce acceptance and growth it is important to understand the potential of impulse purchase in an online shopping environment. This paper critically reviews the factors that influence impulse buying in traditional brick-and-mortar store and the factors that influence online impulse buying and then tries to compare those. First, this paper discusses the concept of impulse purchase as developed over a period of time by several researchers of marketing and psychology. Furthermore it talks about the four basic types of impulse buying and explains them with examples. Second, this paper tries to compare the factors that may influence offline and online impulse purchase and finally it is an attempt to understand to online impulse buying and its international aspect and emerging concepts of online marketing to increase impulse buying online. There is a scope for further research to fill in the gap between literature of impulse buying and literature of online impulse buying. There is a need for more research because it has a lot of potential particularly the online impulse purchase since the online sales are increasing rapidly and are expected to increase at a higher growth rate.