Smell loss is often described as “invisible,” yet its consequences are highly tangible—affecting safety, appetite, social life, and emotional well-being. Because olfactory symptoms can arise from multiple mechanisms, ranging from nasal inflammation to nerve injury or neurodegenerative disease, the same complaint may require markedly different clinical examination and treatment. Effective management, therefore, begins with careful evaluation: not only to confirm the presence and type of olfactory dysfunction but also to identify reversible causes, estimate prognosis, and guide treatment choices with realistic expectations. Clinical assessment typically starts with a detailed history. When did symptoms begin? Was the onset sudden or gradual, and did it follow infection, trauma, medication changes, or toxic exposure? Are symptoms purely quantitative (reduced or absent smell) or qualitative (parosmia and phantosmia)? These distinctions matter, both diagnostically and therapeutically. Examination by an ENT specialist may then identify obstructive or inflammatory disease using nasal endoscopy, particularly in the olfactory cleft. Objective smell testing adds crucial information because self-reports are often unreliable; standardized psychophysical tests can quantify function and support meaningful classification. In selected cases, imaging of the sinuses, olfactory pathways, or brain may be required to clarify uncertain etiologies or exclude structural and neurological causes. In this chapter, we present a structured approach to diagnosing olfactory loss and review the most common causes, including post-viral loss, chronic rhinosinusitis, head injury, medication- and toxin-related effects, congenital anosmia, aging, and neurological disease. We then summarize evidence-based treatments—medical therapy, olfactory training, and surgery where appropriate—and introduce emerging options such as PRP, nutritional support, and future regenerative or implant-based technologies, including management principles for parosmia and phantosmia.