**1. Introduction**

Vitamin D (VD) deficiency is a growing problem worldwide [1]. Due to the wide distribution of the vitamin D receptor in human body, the effect of VD spans beyond calcium-phosphorus and bone metabolism—VD deficiency is associated with higher prevalence of hypertension, diabetes mellitus, and neoplasia [2]. As kidneys play an important role in the metabolism of VD, patients with chronic kidney disease (CKD) are at increased risk for VD deficiency. The aim of our chapter is to demonstrate the clinical implications of vitamin D deficiency in CKD and to outline the possible treatment options in this group of patients. In our review, a stress is laid on clinical trials due to their greater relevance to everyday clinical practice, compared to *in vitro* and animal models.
