The focus of the second issue of 2021 is on vitamin D. Low concentrations of this vitamin are a problem almost everywhere in the world. It is believed that its deficiency is characteristic of many populations, including some latitudes of South Africa, however, regarding vitamin D deficiency and insufficiency one can observe inconsistency and diversity both in the results obtained and in the clinical recommendations of the world’s leading communities. The second can to some extent be considered a consequence of the first: vitamin D as an analyte is difficult to determine, initially because of the short lifetime of its active form. This analyte has in recent years become one of the most popular tests in the major laboratories of the world. There are good reasons for this - there is evidence of the pathogenetic importance of vitamin D in many pathological conditions and the therapeutic effect when prescribing vitamin D preparations. However, there is still no consensus among specialists on the most important issues of diagnosis of hypo- and hypervitaminosis states and the therapeutic effect. Should vitamin D concentrations (metabolites) be determined in individuals outside of epidemiological and clinical studies? Is it necessary to monitor its level when prescribing prophylactic or therapeutic doses of vitamin D drugs, and is there a strict correlation between the dose of vitamin D drugs and its concentration in the blood and clinical effects? Finally, which method and which metabolites to measure. The answers to these questions are equally important for the laboratory diagnostician and the clinician. The articles in this issue reflect the perspective of both treating physicians and laboratory medicine specialists on the practical use of measurements of vitamin D concentrations.