FPGAs are becoming the repository of most or all system IP. Since FPGAs are upgradeable and reprogrammable, they are being updated in the field or over the Internet to correct errors or configure customer preferences on the fly. FPGAs can now be used to build efficient subscription systems. This flexibility has its downside: hackers can tamper with or contract manufacturers can overbuild the design. This paper discusses the various methods for protecting system IP when using FPGAs.