When are member states allowed to impose border controls?
Under Schengen rules, in extenuating circumstances, where a threat to public policy or national security has been identified Member States are permitted to reintroduce temporary border controls.
More detailed information and the list of countries which have temporarily reintroduced border controls. Ensure that you have either your ID or passport in your possession when travelling to these countries. Also bear in mind that even under normal circumstances you may be required to produce one of these documents.