Authorities are predominantly parts of national, state or local police forces under government control. Most of them belong to or represent crime prevention units set up specifically to cooperate with citizens. They are public servants operating under national law and within strict hierarchical structures. Normally their activities comprise advisory services to and cooperation with citizens and other stakeholder groups. In some countries a special police service called community policing has been introduced, with specific police officers assigned to local communities, and with well-defined objectives and procedures. Some countries are in the process of introducing or planning to introduce community policing, in some countries the concept is little known.