The term tumor originated from the Latin word tumere to swell, which means a new growth of certain tissues in which the multiplication of cells is uncontrolled and progressive, and also called a neoplasm [1]. The word tumor is defined as those neoplastic conditions and tumor-like lesions that mimic or may be mistaken for a true neoplasm [2]. Bone tumors are traditionally classified as benign (noncancerous) or malignant (cancerous). Benign bone tumors lack the properties of spreading to local structures or to distant parts of the body, which tend to grow slowly with distinct borders and have a favorable chance for recovery. However, malignant bone tumors have a property of local invasiveness with cortical destruction and soft tissue extension as well as spreading and metastasis to other organs via the bloodstream or the lymphatics with less chance of recovery and survival. Bone tumors are also classified as primary bone tumor which arises in bone or from bone-derived cells and tissues and secondary bone tumor which first starts in another part of the body and metastasize to the skeleton, mainly through the vascular or sometimes lymphatic system. The latter secondary is also called a metastatic bone cancer which is the same type of cancer as the original primary cancer. For example, cancer cells may spread from the primary cancer of the lung to the bone, producing a new secondary cancer. The cancer cells in the bone are identical to those in the lung. Another different type of second cancer can newly occur as a late effect of the first cancer or its treatment, or it may be unrelated to the first cancer. More recently, Double or multiple primary cancers or subsequent neoplasm has been diagnosed in the same individual with increasing incidence. As the population of cancer survivors from their first cancer diagnosis has increased and continues to age, the incidence of second cancers has risen dramatically from 9% of all cancer diagnoses in 1975–1979 to 19% in 2005–2009 [3]. The risk of development varies substantially depending on the age of the patients, the type of initial primary cancer, and its management, including chemotherapy, radiotherapy, and bone marrow transplantation, prevalence of second cancer risk factors, lifestyle and environment, genetic susceptibility to second cancer etiology, and exposure to other cancer risk factors. Precancerous conditions include benign tumors or lesions with abnormal cells, which are associated with a risk of developing into cancer, so-called malignant change or transformation. Higher risk precursors for malignant transformation include Ollier’s disease and Maffucci’s syndrome, familial retinoblastoma syndrome, or Rothmund Thompson syndrome. Moderate risk groups include multiple osteochondromas, Paget’s disease, and radiation osteitis. Lower risk can be associated with bone infarction, long-term chronic osteomyelitis (OM), prosthetic implants, osteogenesis imperfecta, or some benign bone tumors such as fibrous dysplasia (FD), giant cell tumor (GCT), osteoblastoma (OB), and chondroblastoma (CB) [4]. Correct diagnosis of bone tumors has been well known to be difficult to make for a variety of reasons, including the many tumor subtypes, the protean radiologic manifestations, and variable pathologic findings depending on the phases of the tumor, the presence of reactive host bone changes, overlapping findings between benign and malignant tumors, and issues relating to adequacy of biopsy. The aim of this book is to offer the best guidance in negotiating these potential difficulties for a correct diagnosis of bone tumors by a clinical-radiologic-pathologic correlation and to provide adequate options for management and outcomes for each tumor. Early in 1958, Jaffe [5] emphasized the diagnostic triangle of the cooperative approach by the orthopedic surgeon, radiologist, and pathologist to reach a rational diagnosis, and the concept is still valuable in diagnosis of bone tumors (Fig. 1.1a). They should share their points of view on a bone lesion, including clinical factors such as patient age, location in a particular bone or part of a bone, solitary or multiple lesions, and other preexisting conditions for a rational diagnosis as well as radiologic and pathologic findings. For a simple example, a differential diagnosis of osteofibrous dysplasia from FD cannot be attained only with a microscopic finding of irregular, curvilinear trabeculae of woven bone with osteoblastic rimming and intervening bland spindle cells in some cases. However, it is easily distinguished by adding only one radiograph with an eccentric cortex-based lesion exclusively in the tibia and/or fibula with an open growth plate indicating a young child (Fig. 1.1b). Vice versa, a diagnosis of fibrous dysplasia can also be easily made by radiographic findings of a concentric intramedullary lesion with a “ground-glass” density in the midshaft of other long bones (Fig. 1.1c). Even such an approach cannot avoid all diagnostic errors, but it can certainly make them fewer and less glaring. The recent advent of molecular diagnostic techniques has expanded this diagram into a diagnostic quadrangle to reduce diagnostic errors [6], but the technique is not covered in this edition of the books (Vols. 1 & 2). These books provide a comprehensive conventional guide to the radiologic and pathologic diagnosis, biopsy technique, as well as clinical outcomes and radiologic follow-up results after conservative or surgical management in primary benign and malignant bone tumors. In addition, histologic findings of callus tissue caused by fatigue or stress fracture and even pathologic fracture that can make a diagnosis more complicated will be presented by the age of the callus to reduce the wrong diagnosis in its own chapter. The emphasis throughout the books is on an integrated team approach to diagnosis and management, highlighting the important role of clinical, radiologic, and pathologic correlation in reducing the possibility of diagnostic error and avoiding undertreatment or overtreatment. Appearances on the full range of imaging studies are illustrated, including plain radiographs, CT scans, MRI, bone scans, and SPECT CT as well as PET/CT scans. Attention is also drawn to the role of long-term follow-up radiographs, ensuring if the diagnosis with management is reasonable. The incidence, favorite ages, and locations described of each tumor are based on the published data mainly from several major centers such as Mayo Clinic, Rizzoli Institution, or Massachusetts General Hospital, as well as published books edited by Campanacci M, Unni KK, Mirra JM, Czerniak B, or others. Survival rates of primary malignant bone tumors are based on the recent analysis of the National Cancer Institute Surveillance, Epidemiology, and End Results (SEER) program as well as of Mayo Clinic, Rizzoli Institution, MD Anderson Cancer Center group, and others.