In this book a very mature and precise review is done to the photoluminescence up-conversion phenomenon. After an introduction to the semiconductors and some of optical properties of them such as photoluminescence and photoluminescence up-conversion, the mechanisms are studied. Furthermore, the factors which are effective in the enhancement of photoluminescence up-conversion are indicated and analyzed.The conclusions of our studies show that the removal of the linear momentum's conservation bond in the heterostructures has a very prodigious effect on the efficiency of this optic phenomenon.