The lymphatic system is responsible for the return of protein-rich interstitial fluid to the circulation and is also involved in the body’s defense and fat absorption. Due to damage to the lymphatic system or excessive production of lymph fluid in the body, the disease called lymphedema occurs. The type of lymphedema caused by genetic mutations that cause underdevelopment of lymphatic vessels and impaired lymphatic drainage capacity is called primary lymphedema, and the type that occurs as a result of damage to the previously normal lymphatic system due to causes such as recurrent infection, venous diseases, trauma, radiation, endocrine diseases, immobility is called secondary lymphedema. In our country, lymphedema that develops in the arm after breast cancer surgery is most common. One of the most important steps in the effective treatment of lymphedema is early diagnosis. There are no universally accepted criteria for the diagnosis of the disease. Taking patient history, volumetric measurement, circumference measurement, imaging methods (bioimpedance analysis, lymphoscintigraphy, magnetic resonance imaging, computed tomography, ultrasound) are among the evaluation methods used in patients with lymphedema. The International Society of Lymphology has divided the treatment of lymphedema into 2 types: operative and conservative methods. Both treatments include skin care and patient education and these methods are used in lymphedema. In this section of the book, the anatomy of the lymphatic system, the pathophysiology of lymphedema, the assessment of lymphedema, and current lymphedema treatments are discussed.