The lymphatic system is arguably the most neglected bodily system. As a result, its contribution to human health and disease is not well understood. In this review, the clinical approaches based on new knowledge and developments of the lymphatic system are covered. The lymphatic system has 3 major functions: (1) the preservation of fluid balance; (2) a nutritional function, as intestinal lymphatics are responsible for fat absorption; and (3) host defense. Lymph vessels return the capillary ultrafiltrate and escaped plasma proteins from most tissues back, ultimately, to the blood circulation. Hence, lymphatics are responsible for maintaining tissue (and plasma) volume homeostasis. Impaired lymph drainage results in peripheral edema (lymphedema) and may have more far-reaching effects on cardiovascular disease, in particular hypertension and atherosclerosis. Lymphatics have an important immune surveillance function, as they represent the principal route of transport from tissues for antigen and immune cells. Intestinal lymphatics (lacteals) are responsible for most fat absorption, first documented by Gaspare Aselli in 1627, when the lymphatic system was discovered. A relationship between fat and lymphatics may exist well beyond the gut alone. Fat deposition is a defining clinical characteristic of lymphedema. Suction-assisted lipectomy of lymphedema has shown the swelling is not just fluid but is dominated by fat. Lymphatics are the preferred route for the metastatic spread of cancer. Accordingly, the lymphatic system may be important for defense against cancer by generating immune responses to malignant cell antigens. Preventing lymphatic entry and propagation of malignant metastasis would effectively render the cancer nonfatal. As one can see, the lymphatic circulation is fundamentally important to cardiovascular disease, infection and immunity, cancer, and, in all likelihood, obesity — 4 of the major challenges to health care in the 21st century.