Vitamin D and vitamin D deficiency strongly interact with different pathogenic mechanisms in COPD. Prevalence of vitamin D deficiency is particularly high in COPD patients, increases with the severity of COPD, and is closely associated with osteoporosis prevalence. Adequate calcium and vitamin D supplementation in COPD patients with documented deficiencies reduces the risk for falling and osteoporotic fractures, may indirectly reduce morbidity, and may potentially prevent the further deterioration of pulmonary function. Apart from the proven beneficial effects of vitamin D supplements on bone and muscle tissue, many epidemiological studies have putatively linked vitamin D deficiency with a higher risk for cardiovascular, inflammatory and infectious diseases, and cancer, diseases known to be associated with and to contribute significantly to the phenotypic presentation of COPD patients. Different animal and human studies have provided considerable evidence on how vitamin D may affect these processes. The burning question in COPD is whether prevention of vitamin D deficiency or adequate supplementation may reverse the natural course of the disease.