Bone is permanently renewed by the coordinated actions of bone-resorbing osteoclasts and bone-forming osteoblasts, which model and remodel bone structure during growth and adult life. The origin of osteoblastic cells (osteoblasts, osteocytes and bone-lining cells) differs from that of osteoclasts, but both cell groups communicate with each other using cytokines and cell-cell contact as to optimally maintain bone homeostasis. This communication in many ways uses the same players as the communication between cells in the immune system. During acute life-threatening illness massive bone resorption is the rule, while bone formation is suppressed. During chronic illness, the balance between bone formation and bone resorption also shifts, frequently resulting in decreased bone mass and density. Several factors may contribute to the osteopenia that accompanies chronic illness, the most important being undernutrition and low body weight, inflammatory cytokines, disorders of the neuroendocrine axis (growth hormone/IGF-1 disturbances, thyroid and gonadal deficiency), immobilization, and the long-term use of glucocorticoids. Their combined effects not only influence the generation and activity of all bone cells involved, but probably also regulate their life span by apoptotic mechanisms. Osteopenia or even osteoporosis and bone fragility, and before puberty also decreased linear growth and lower peak bone mass are therefore frequent consequences of chronic illnesses.