Nowadays, the complexity and size of software systems is proliferating. These factors have various pros and cons. On one side where they lead to better performance and satisfactory results, on the other side they lead to high testing cost , wacky results , poor quality and non-reliability of the product. These problems have one root cause which is referred to as defects in the software systems Predicting these defects can not only rule out the cons but can also boost up the pros. Various techniques are present for the same which are reviewed in depth in this paper. Moreover, a comparison of these techniques is also done to throw a lime light on those which provide the best results.