Osteosarcoma (OS) is the most common primary bone tumour in dogs. Current treatment methods are ineffective at preventing the development of pulmonary metastasis, a clinical reality for most canine patients and the most common cause of mortality. To improve patient outcomes, it is imperative that we improve our understanding of metastasis biology and discover new therapeutic targets. This thesis encompasses three studies that used in vitro and in vivo techniques, as well as patient samples, to expand our knowledge of OS biology. The first study explored Hippo-YAP signalling in vitro and found that YAP mediates migration and chemoresistance. The second study explored the use of the YAP inhibitor, verteporfin, for the treatment of advanced disease and characterized its impact on the lung microenvironment throughout metastasis development. This study found that the lung’s innate immune cells increase throughout metastatic progression, while there were no changes in lung stiffness or collagen deposition. Verteporfin decreased pulmonary metastatic burden and innate immune cell infiltration, while moderately enhancing lung stiffness throughout metastatic progression. The third study aimed to find new protein signatures by characterizing the protein cargo of extracellular vesicles (EVs) derived from canine OS tissue explants. This novel pipeline demonstrated that EVs could be isolated from non- malignant and OS tumour tissue. Upon characterizing the EV cargo, I identified a protein signature related to protein metabolism. The mRNA coding for two of the proteins, PSMA7 and PSMD14, were associated with prognosis. In vitro targeting of PSMD14 with the drug capzimin induces apoptosis and reduces clonogenic survival, proliferation, and migration in two canine OS cell lines derived from metastatic nodules. Future work could elucidate the mechanism of action of verteporfin, investigate the role of innate immune cells in OS metastasis, expand the EV pipeline to include the secondary tumour with more patient samples, and explore the prognostic and predictive value of the proteins identified in patient’s plasma/serum samples.