The term vitamin D refers to a family of secosteroid compounds derived from cholesterol that are best known for facilitating intestinal absorption of calcium and phosphate required for normal bone development and homeostasis. There are two major forms of vitamin D: vitamin D2, known as ergocalciferol, which is derived from plants, and vitamin D3, known as cholecalciferol and which is derived from animal tissues. Humans generate vitamin D from 7dehydrocholesterol in the deep epidermis following exposure to natural ultraviolet B (UVB) radiation in sunlight. In the absence of supplementation, most vitamin D in humans is dermal in origin and dependent on adequate sunlight exposure, with more pigmented people requiring longer exposure than those who are lightly pigmented to achieve similar serum levels of vitamin D. Fatty fish and fortified foods can augment skin-based synthesis, but dietary vitamin D alone is often insufficient to achieve adequate doses. 1-6 Given indoor work-life routines and current recommendations to limit unprotected sunlight exposure, it is perhaps not surprising that billions of people around the world are believed to be vitamin D deficient, defined as a serum level of less than ≤20 ng/mL (50 nmol/L). Although accurate estimates of vitamin D deficiency are not available for most countries, 7 a survey of 4495 US citizens tested as part of a 2005-2006 National Health and Nutrition Examination Survey identified vitamin D deficiency in approximately 40% of adults, with much higher rates in Latinos (~70%) and African-Americans (~80%). 8 Rates of vitamin D deficiency in many parts of Asia, India, The Middle East, and African are believed to be in excess of 90%. 9,10 In the extreme, vitamin D deficiency results in rickets in children and osteomalacia in adults. However, increasing epidemiological evidence points to important roles for vitamin D in normal functioning of the cardiovascular, nervous, endocrine, and immune systems. In fact, a number of epidemiological studies have associated vitamin D deficiency with statistically higher rates of all-cause mortality, of cancer, and of cardiovascular, neurodegenerative, metabolic, and autoimmune diseaseincluding noninfectious uveitis and scleritis. [1][2][3][4][5][6][11][12][13][14][15][16][17][18][19][20][21][22] Specific forms of uveitis linked to vitamin D deficiency include noninfectious anterior uveitis, 18,19 anterior uveitis in the setting of juvenile idiopathic arthritis, 17 Behçet's-associated uveitis, 15,21 and uveitis in patients with Vogt-Koyanagi-Harada disease. 20,22 Of note, Chiu et al. 13 recently performed a prospective case-control study of 151 patients with noninfectious uveitis seen at two tertiary centers in Victoria, Australia, from January through August 2017. The researchers found that subjects with active uveitis tended to have lower serum vitamin D levels than patients with inactive uveitis (46 vs 64 nmol/L; p < .001), that these levels in patients with active uveitis were lower than those observed in the local non-uveitic populat...