Flavor is often described as a property of food itself, yet it is largely constructed by the brain through sensory integration. While taste contributes basic qualities such as sweetness or bitterness, it is the sense of smell—particularly retronasal smell—that provides most of the richness, nuance, and identity of what we experience when eating. Without retronasal olfaction, food quickly loses its complexity, becoming nutritionally functional rather than pleasurable. Understanding retronasal smell is therefore essential for understanding flavor. This chapter explores why humans are especially skilled at perceiving flavor and how this ability emerged through evolution. We describe how anatomical changes, including the development of a hard palate and altered airflow between the mouth and nose, enabled odorants released during eating to reach the olfactory receptors. These changes distinguish human flavor perception from that of many other mammals and help explain why retronasal smell plays such a dominant role in food evaluation. The chapter also introduces the concept of the duality of smell—orthonasal and retronasal—and explains how the same odor can be perceived differently depending on its route into the nose. In this chapter, we examine the physical and chemical processes that release odorants during chewing, swallowing, and salivation, and how these signals are integrated with taste and texture to create the multisensory experience of flavor. We also address phenomena such as odor referral, sensory interactions, and individual variability, and include practical demonstrations and patient perspectives to illustrate the real-world consequences of altered retronasal perception. Together, these sections provide a foundation for understanding both normal flavor perception and the profound impact of smell loss on eating and quality of life.