Vitamin D is a fat-soluble hormone that can be synthesised by the body from sunlight or some dietary sources. Deficiency of vitamin D leads to the production of poorly mineralised bone and dentin. Guidance for diagnosis of active, healed and recurrent episodes of deficiency in individuals of all ages using macroscopic, radiographic and microscopic techniques are provided using the approaches set out in Chapter 2 and consideration of the basic biological mechanisms that underlie lesion formation discussed in Chapter 3. Study of deficiency in the past can tell us about cultural practices, environmental pollution, or other factors that acted to limit, at the individual or community level, exposure of the skin to sufficient natural light to enable adequate synthesis of vitamin D. A series of bullet points are used to summarise the core concepts covered.