Thermoelectric is a device that converts heat into electricity. As thermodynamically it is not possible to make device which is 100 percent efficient, some amount of energy is wasted in the form of heat. Thermoelectric materials can play a major role in harnessing such waste energy. Although thermoelectric is a useful device still its efficiency is not good enough for commercialization. Therefore, lots of research have been carried out in finding out the best possible material, device geometry etc. There are thousands of papers describing various optimization processes. The present work reviews the basics of thermoelectric device parameters which determine the performance of the device and how to control these parameters for better thermoelectric efficiency. The efforts made to optimize parameters like power factor, thermal conductivity etc. have been summarized. Experimental results have been described with examples. Highest reported ZT values of various materials have been presented in this review.