Osteosarcomas are the most common primary neoplasia of the appendicular skeleton in dogs. Neoplastic cells produce bone, osteoid or both. The exact cause for its occurrence is not fully known. More recently, it has been associated with mutations in certain genes. Most often osteosarcoma occurs in large and gigantic dog breeds of middle and older age. The most common places for osteosarcoma to occur are those who carry weight like proximal humerus, distal radius, femur, distal ulna and proximal tibia. Usually osteosarcomas arise inside the bone, especially in metaphyseal region of long bones. There are also periosteal and parosteal osteosarcomas. Osteosarcomas are very heterogeneous neoplasms with various histological patterns. According to World Health Organization we distinguish 6 subtypes: osteoblastic, hondroblastic, fibroblastic, telangiectatic, poorly diferentiated and giant cell rich osteosarcoma. They are classified according to the dominant histological pattern. Regardless of the subtype, osteosarcoma is very progressive neoplasia and early metastases are often recorded, especially in the lungs. It is diagnosed through anamnesis, clinical examination and diagnostic imaging. In order for the diagnosis to be completely accurate and to determine the degree of neoplasia, cytopathological and histopathological examinations are necessary.