Oral cancer is one of the drastic human cancers because of its aggressiveness and has high mortality rate. Epithelial mesenchymal transition is an important event in the development of cancer. It is the process where the epithelial cells loose cohesiveness and their cellular polarity and gain the migratory and invasive properties to become like mesenchymal cells. It is also known as the biological process which allows the epithelial cells to interact with the basement membrane to undergo multiple biochemical changes. So here the fully differentiated epithelial cell loses all their character .There are three types of epithelial mesenchymal transition. First type occurs in the formation of embryo. This is known as developmental EMT. Second type of epithelial mesenchymal transition occurs in wound healing. It can be also associated with inflammation and fibrosis. Type three occurs in the development and progression of cancer causing the cells to become mobile, and lead to the activation of proteases it can break the basement membrane too. In this book we explain the importance of epithelial mesenchymal transition in oral squamous cell carcinoma.