Bone is that part of the skeletal system which caters for structural stability and protection of the internal organs. It is composed mainly of cells and a collagenous extracellular matrix (type 1 collagen) called osteoid which becomes mineralised by the deposition of calcium hydroxyapatite crystals. It is thus not surprising that another main function of bone is to act as a mineral resevoir. In this way bone is used up according to the body's blood calcium levels; when calcium levels are low bone resorption increases and vice versa, so bone is a dynamic material which is constantly being made and broken down. There are various factors that influence bone density, such as age, genetic factors and others all of which mainly influence the stages of bone remodelling. In this book, various aspects regarding bones were discussed including: the biology of the bone, physiology of Vitamin D, parathyroid hormone and calcitonin regulating Calcium levels and clinical correlates involving deficiency of Vitamin D and Calcium.