The dramatic rise in mobile applications has greatly increased threats to the security and privacy of users. Security mechanisms on mobile devices are currently limited, so users need more expressive ways to ensure that downloaded mobile applications do not act maliciously. Policy-specification languages were created for this purpose; they allow the enforcement of user-defined policies on third-party applications. We have implemented LoPSiL, a location-based policy-specification language for mobile devices. This article describes LoPSiL’s design and implementation, several example policies, and experiments that demonstrate LoPSiL’s viability for enforcing policies on mobile devices.