Vitamin D insufficiency is an extremely common condition particularly in the older Irish population. This is a consequence of Ireland's geographical position and climate. A recent study showed more than 75% of a cohort of older Irish females had vitamin D insufficiency.We outline the definition of vitamin D insufficiency and deficiency, its sources and metabolism as well as the clinical consequences of deficiency. We explore the current guidelines and discuss the pitfalls in the management of vitamin D replacement and in particular address recent Irish data on the feasibility and efficacy of intramuscular treatment.Vitamin D is important for calcium homeostasis and bone metabolism and reduced levels lead to osteomalacia, exacerbate osteoporosis and increase the risk of falls. Evidence is emerging that vitamin D has a role beyond musculoskeletal health and may impact on the cardiovascular and autoimmune systems, as well as the risk of malignancy.