Policies are increasingly used to govern the behaviour of complex distributed systems. Most policy models that allow policy composition, to address the complexity of policies, are only concerned with structural composition. In this paper we argue that it is natural to compose policies also along the temporal axis, i.e. express policies that can dynamically change over time or on the occurrence of events. We present a dynamic policy frame-work that has a sound semantics in Interval Temporal Logic and allows both structural and temporal composition.

We provide examples of authorisation, delegation and obligation policies that can be activity-based, state-based or historybased, i.e. expressing decisions on the history of execution. Examples for the composition of policies show how the framework can be used to express policies for systems that operate in an environment that is characterised by uncertainty. Finally tool-support for the specification and analysis of dynamic policies is presented.