A range of diphosphonates are labelled with Tc-99m for imaging the skeleton, including Tc-99m-MDP (methylene diphosphonate), Tc-99m-HDP (hydroxy-diphosphonate) and Tc-99m-DPD (diphosphono-propanodicarboxylic acid). Tc-99m-diphosphonates accumulate in the osteoid seam of bone andportray osteoblastic activity without accumulating in osteoblasts or osteoclasts. Bone scans are non-specific in that, in general, tracer accumulates in bone disorders - whatever their cause - in response to the increased osteoblastic activity associated with healing. Bone scintigraphy, therefore, has a wide range of clinical applications, the most frequent of which is the detection of bone metastases. Other indications are unexplained bone pain (often in an orthopaedic setting), suspected infection and/or loosening of joint prostheses, inflammatory arthritis, occult fractures and Paget's disease. The commonest bone disease encountered in adult bone scintigraphy is degenerative joint disease. The paediatric bone scan is dominated by increased uptake in growth plates. Indications in children include primary bone tumours, osteomyelitis, avascular necrosis, condylar hyperplasia and fractures, especially in non-accidental injury.