Splenic hemangiosarcoma is a devastating cancer of older, large-breed dogs with short survival times even with treatment. Similarly, human angiosarcoma is a rare but devastating cancer with short survival times. Similarities exist between hemangiosarcoma and angiosarcoma, but we have limited information on what drives metastasis in these tumors. The objective of this project is to identify differences between dogs with splenic hemangiosarcoma that have metastasis and those that do not and to compare these findings to human angiosarcoma to identify new areas for treatment. This research stands to benefit all dogs that are affected by hemangiosarcoma. Golden Retrievers, Labrador Retrievers, and German Shepherd Dogs are commonly affected by hemangiosarcoma, but many other breeds and mixed breed dogs can be affected. The results of this work will also help people affected by angiosarcoma.