The proliferation of counterspace weapons across the globe often calls into question what can be done to best protect satellites from attack. This analysis from the CSIS Aerospace Security Project addresses different methods and technologies that can be used by the United States government, and others, to deter adversaries from attack. A wide range of active and passive defenses are available to protect space systems and the ground infrastructure they depend upon from different types of threats. This report captures a range of active and passive defenses that are theoretically possible and discusses the advantages and limitations of each. A group of technical space and national security experts supported the analysis by working through several plausible scenarios that explore a range of defenses that may be needed, concepts for employing different types of defenses, and how defensive actions in space may be perceived by others. These scenarios and the findings that resulted from subsequent conversations with experts are reported in the penultimate chapter of the report. Finally, the CSIS Aerospace Security Project team offers conclusions drawn from the analysis, actionable recommendations for policymakers, and additional research topics to be explored in future work.