Vitamin D, or 1,25-dihydroxy vitamin D (1,25[OH]D), in its activated form, has long been recognized as a critical mediator in bone health. New research has identified 1,25(OH)D as also vital for respiratory health. Owing to its intrinsic anti-inflammatory properties, 1,25(OH)D may be very important in people with asthma. This review article seeks to evaluate the current literature to delineate the potential mechanisms of action by which 1,25(OH)D affects asthma. We summarize the evidence that 1,25(OH)D has receptors in multiple lung cell types and acts to abrogate asthma by several mechanisms: promoting lung immunity, decreasing inflammation, slowing cell cycling, reducing hyperplasia, and enhancing the effects of exogenous steroids. Put together, there is compelling evidence for the role of vitamin D in asthma.