Abstract: Animals detect physical and chemical changes in their environment by using their sensory systems. Olfactory system is one of them and its job is to detect chemicals in the environment. It helps them in finding foods, mating, avoiding poisons and escaping from dangers. Although, the survival of many animals depends on their olfactory system, we know very little about olfaction, comparing to other senses. For example, there is not any agreement in the scientific community about the mechanism of interaction between odorant and odor-receptors, there are some theories and an ongoing debate. This is due to the complex nature of olfaction. Half of this complexity arises from the diversity of chemicals in the environment, and the other half arises from the large number of odor-receptors. In this talk, we first review the olfactory pathway and then compare it to the well known sensory system of vision. After that, we investigate two major theories of olfaction, shape theory and vibrational theory of olfaction and we finally present our results, in which we investigated the odorant odor-receptor relation in Drosophila (fruit fly).