Computers and people live in a world governed by policy. At the lowest level, policies determine how information flows within networks; at the highest level, they describe how users' personal information is shared across applications. Of course, end-users, as policy authors, make mistakes: rules can have unintended consequences and multiple policies can interact in ways that their authors didn't intend. Users can benefit from tools to help them understand the policies they write and maintain. Policy analysis refers to rigorous methods for detecting these situations before they cause harm.