A significant limitation to the use of viruses as systemic vectors is the susceptibility of the vector to inactivation and clearance by various blood components. Despite much focus on antibodies as the primary neutralizing molecules in blood, other mechanisms inactivate and clear virus particles from the bloodstream in both naïve and pre-immune hosts. This review provides an overview of the major blood components that interact with enveloped viruses. The mechanisms of action of these blood components by which virus particles are inactivated are also discussed. In addition, important blood components that act as barriers to the systemic delivery of therapeutic viruses are identified, and recent advances in overcoming these barriers are highlighted. Particular attention is given to the field of oncolytic virotherapy in which adequate intravenous virus delivery is critical for therapeutic success.