The WHO [10] has made several recommendations concerning obesity. One of these addresses the need for the development and validation of new and ex- isting techniques. In this chapter, we have briefly reviewed many of the existing techniques and their limitations when applied to obese persons. In support of this WHO recommendation, it is clear that existing techniques are not applica- ble to many obese who are in great need of this technology. This limitation also affects our ability to determine the real prevalence of obesity because the cur- rent methods are not applicable to large epidemiological and clinical studies. Obviously much work is yet to be done.