In this white paper, we introduce the concept and operating principles of passive radar systems. We review the basics of bistatic radars and introduce the concept of emitters of opportunity (EoO) and their ideal qualities. We discuss how satellites can be used as EoOs and evaluate the role of satellite illumination and forward scattering. We also con-sider how passive radar can be applied to stealth technologies and highlight examples of operational passive radar systems. The white paper concludes with a review of technologies that can be used during research, development, testing and evaluation (RDT&E) of passive radar systems.