Osteosarcoma, (OS) is the most common malignant bone tumour in dogs. OS occurs more frequently in the appendicular skeleton than the axial, and predilection site is the metaphyseal region of long bones. The aetiology behind OS is still unknown but several risk factors have been identified, such as age and body size. Mainly old, large and giant dogs are affected and some breeds like Rottweiler and Grand danois show an increased susceptibility. This higher prevalence among certain breeds has contributed to studies on inheritability of OS. OS in men and dogs share similarities such as predilection sites, malignancy and tendency to metastasize. There are also genetic alterations in patients with OS that has shown similar features in these two species, and it has made the dog to an osteosarcoma research model. Tumour genesis is often due to alterations of tumour suppressor genes like p53 and retinoblastoma, Rb. These genes are normally protecting the body from altered cells by 3 inhibiting mitosis, either for cell reparation or to induce apoptosis. Changes in tumour suppressor- and oncogenes have a central role in tumour genesis. OS has shown to exhibit recurrent alterations of these genes in both men and dog. Still it remains unknown how these changes in detail are affecting initiation, progression and prognosis of this tumour.