Light micrograph of Ewing sarcoma in the rib of a 12 year old male patient. On the left, the tumour is seen permeating the marrow spaces of the cortical bone. On the extreme right, there are delicate spicules of new reactive woven bone. Ewing sarcoma (ES) is an aggressive, undifferentiated, small round cell sarcoma of bone and soft tissues, predominantly arising in children and young adults. It resembles and shares morphologic, immunohistochemical and genetic features with Askin tumour (involving the chest wall) and peripheral primitive neuroectodermal tumour. They are collectively referred to as Ewing sarcoma family of tumours. ES most commonly involves long bones (femur, tibia, humerus, and fibula), pelvis, chest wall (ribs), vertebrae and clavicle.