In vitro dissolution has been extensively used as a quality control tool for solid oral dosage forms. In several cases, however, it is not known whether one can predict the in vivo performance of these products from in vitro dissolution data. In an effort to minimize unnecessary human testing, investigations of in vitro / in vivo correlations (IVIVC) between in vitro dissolution and in vivo bioavailability are increasingly becoming an integral part of extended release (ER) drug product development. This increased activity in developing IVIVCs indicates the value of IVIVCs to the pharmaceutical industry. Because of the scientific interest and the associated utility of IVIVC as a valuable tool, the US Food and Drug Administration has published a Guidance in September 1997, entitled Extended Release Oral Dosage Forms: Development, rapidity in Drug development can be achieved by researchers on finding a mathematical link between bioavailability and dissolution testing which leads to the concept of in vitro -in vivo correlation (IVIVC). IVIVC is a mathematical model that can be used to estimate in vivo behavior from its in vitro performance. Among all the five levels of correlation, Level A correlation is widely accepted by the regulatory agencies. Biopharmaceutical Classification System (BCS) explains the suitability of IVIVC. Dissolution method design plays a pivotal role in the estimation of correlations. Applications of IVIVC ranges from drug and product development, their scale up and post approval changes. Hence, IVIVC should be considered as an important tool in drug development.