Vitamin D insufficiency is a term that has been used to describe the finding of biochemical evidence of deficiency, without obvious clinical signs or symptoms, such as rickets or osteomalacia. The condition is most commonly diagnosed by a serum 25-hydroxyvitamin D below 40 nmol/L (16 µg/L). This paper reviews North American studies addressing the prevalence of the problem, and the growing body of evidence that vitamin D insufficiency predisposes individuals to poor bone and muscle health. The term insufficiency is somewhat misleading, as patients with this condition are really just part of the spectrum of vitamin D deficiency. If the more generous definition of this condition is used (serum 25-hydroxyvitamin D < 80 nmol/L), a much larger proportion of the population has the problem. The response to vitamin D supplementation in clinical trials suggests current recommendations for dietary intake of this vitamin are too low and that a higher adequate intake should be recommended.