In this Paper we will discuss about birthday attack which is mainly based on birthday problem .Birthday problem is basically a probability problem in which situation is, if there are 23 people in a room, the probability of two people having birthday on same date is slightly more than 0.50. If there are 30 people, the probability increases to 0.70. This is called the birthday paradox. We use this problem in a generalized form in cryptography which we call as Birthday attack. Consider a variation of the same problem, suppose there are two rooms, each with 30 people, what is the probability that someone in the first room has the same birthday as someone in the second room? Birthday attack can even be used to find collisions for hash functions if the output of the hash function is not sufficiently large. In this paper we shall see what hash function is and why birthday attack is so important for hash function. The paper has 3 main segments, 1) Introduction – Two sub parts a) Terminology – Definition of the terms used in birthday attack. b) Method – What is birthday attack Algorithm? What is its use in real life? A discussion on variation of birthday attack problem. 2) Background-In this section we will discuss extensively about past work. 3) Conclusion