Vitamin D deficiency is very common in Germany because the synthesis of Vitamin D in the skin depends on exposure to radiation from sunlight (ultraviolet light type B). In Central Europe, vitamin D synthesis in skin is significantly diminished during the 6-month winter period. Whereas a slight lack of vitamin D (vitamin D insufficiency) results in osteoporosis (decrease in bone mass), a major lack of vitamin D (vitamin D deficiency) leads to osteomalacia, a mineralization defect of the bone. Before the skeletal consequences of vitamin D deficiency take place, neuromuscular functional disorders may already occur. An early diagnosis of vitamin D deficiency is important. In this respect, laboratory abnormalities are especially of great importance. For example, an increase in serum parathyroid hormone concentration is a very early sign. The presence of vitamin D deficiency can be proven by measuring the serum concentration of 25-hydroxyvitamin D. During the treatment of vitamin D deficiency with drugs consisting of vitamin D (vitamin D supplements), the measurement of serum 25-hydroxyvitamin D is very useful for monitoring of therapy and follow up.