Advisory structures with independent status can be established and mandated by executive or legislative bodies in order to provide scientific advice. Governmental agencies play a key role in providing advice on technical and regulatory issues. Their advice is often requested in emergencies or crisis situations. Permanent advisory committees and councils are established to provide expertise and advice on specific topics, usually for long-term policy development. To answer a specific need, an ad hoc committee can be created with a clear remit and on a limited time basis.