Vitamin D deficiency and insulin resistance frequently coexist, and a growing body of evidence links low vitamin D status to obesity, prediabetes, type 2 diabetes, metabolic syndrome, metabolic dysfunction-associated fatty liver disease, polyendocrine metabolic ovarian syndrome, and cardiovascular risk. Yet causality, mechanisms, and the clinical benefit of supplementation remain debated. This Special Issue brings together eleven original research articles, reviews, and a systematic review exploring the relationship between vitamin D and insulin resistance across diverse populations, including children, pregnant women, and adults with obesity. Contributions examine biological mechanisms linking vitamin D to glucose metabolism and adipose tissue, report on associations observed in clinical cohorts from several countries and critically assess the evidence for vitamin D supplementation as a strategy to improve insulin sensitivity and prevent type 2 diabetes. By gathering this work in one place, the Special Issue offers researchers and clinicians a current, integrated view of an actively evolving field and highlights open questions for future study, particularly the design of supplementation trials capable of establishing causal effects.