Summary: Vitamin D is essential in mineral metabolism and has potent pleiotropic extraskeletal effects on immunity, the neurological system, cardiovascular disease, and antineoplastic activity. It is possible due to the widespread distribution of 1α-hydroxylase and the vitamin D receptor in most tissues that, therefore, can convert 25-OH-D to 1,25(OH)2D, which act in an endo-, autoor paracrine manner. Vitamin D deficiency has been associated with the worsening of various metabolic, cardiovascular, and autoimmune disorders, malignancies, and increased mortality. In patients with advanced chronic kidney disease (CKD) and those on hemodialysis, in addition to profoundly impaired 1,25-OH-2D renal synthesis, there is also a widespread prevalence of vitamin D deficiency, in winter reaching 80–90%, with dramatically low serum 25-OH-D, posing a risk of osteomalacia. Deeply reduced 25-OH-D levels are also highly prevalent in kidney transplant recipients, which seem to play some role in the occurrence of post-transplantation bone disease and the cardiovascular, immunologic, and infectious complications after kidney transplantation. In this review, the vitamin D metabolism and the prevalence of 25-OH-D deficiency in advanced CKD, patients on hemodialysis, and those after kidney transplantation are shortly discussed. Then, the current vitamin D status monitoring and supplementation recommendations are presented.