Smell is one of our most fundamental yet least understood senses. It quietly shapes how we experience food, evaluate our surroundings, detect danger, and form emotional memories—often without conscious effort. When smell functions normally, it is easy to overlook its complexity. When it is altered or lost, however, its importance becomes immediately clear. To understand why smell disorders can be so disruptive, it is first necessary to understand what a smell actually is, how the olfactory system works, and how the brain interprets olfactory information. In this chapter, we introduce the biological foundations of the sense of smell, from the chemical nature of odor molecules to the specialized receptors that detect them in the nose. We explain how smell reaches the brain through two distinct routes—orthonasal and retronasal—and why this dual pathway is essential for both environmental awareness and the perception of flavor. The chapter also outlines the basic anatomy of the olfactory system, including the olfactory nerve, bulb, and cortical processing areas, highlighting how fragile this system can be and why it is vulnerable to injury. Building on this foundation, the chapter describes the most common forms of olfactory dysfunction. These include reduced or absent smell (hyposmia and anosmia), as well as qualitative distortions, such as parosmia and phantosmia. Throughout, we connect mechanisms to lived experience, emphasizing how changes in smell perception affect daily life, emotional well-being, and eating behavior. Together, this chapter provides the conceptual framework needed to understand both smell loss and recovery in later sections of the book.