Human society runs on trust. We all trust millions of people, organizations, and systems every day -- and we do it so easily that we barely notice. But in any system of trust, there is an alternative, parasitic, strategy that involves abusing that trust. Making sure those defectors don't destroy the cooperative systems they're abusing is an age-old problem, one that we've solved through morals and ethics, laws, and all sort of security technologies. Understanding how these all work -- and fail -- is essential to understanding the problems we face in today's increasingly technological and interconnected world.