Hepatitis Delta Virus (HDV) is a satellite of Hepatitis B Virus (HBV), and can only reproduce in hepatocytes which are simultaneously infected with HBV. Patients chronically infected with both HBV and HDV are more likely to experience liver failure, hepatocellular carcinoma, and cirrhosis than those infected with HBV alone. We develop a model of HBV-HDV infection to explore the roles of the two infections, their interactions, and the immune response in patient outcomes.