A processing device (10) includes a policy evaluation module (131) for evaluating policies associated with an item of data or an application and a dynamic context determination module (133) for determining contextual information associated with the current context of operation of the device and for providing the thus determined contextual information to the policy evaluation module. The device (10) further includes a policy enforcement module (135) for enforcing the evaluation specified by the policy evaluation module (131), wherein the device is operable to cause the policy evaluation module to evaluate a policy associated with an item of data or an application whenever the associated item of data or application is invoked and, additionally, whilst the associated item of data or application is active on the device and a notification of a change in the determined contextual information is received by the policy evaluation module.