There are two main European regulations covering the mutual recognition of professional qualifications: Directive 89/48/EEC and Directive 92/51/EEC.
Directive 89/48/EEC covers the mutual recognition of qualifications in recognised professions that require a University degree or equivalent. This is implemented in the UK by The European Communities (Recognition of Professional Qualifications) Regulations 1991 and by similar regulations in other member states.
Directive 92/51/EEC covers the mutual recognition of qualifications in professions regulated below degree level. This is implemented in the UK by the The European Communities (Recognition of Professional Qualifications) (Second General System) Regulations 2002 and by similar regulations in other member states.
The regulations cover all fifteen member states of the European Union plus Norway, Iceland and Lichtenstein. They mean that any form of work other than those covered by the Transitional Measures Directive (Directive 99/42/EC, covering crafts and trades people such as hairdressers and construction workers) or the Sectoral Directives (dental practice, medicine, nursing, pharmacy, veterinary practice, and architecture - this was the original method of achieving mutual recognition but proved too slow) that would normally be restricted in a member state to people who had gained a professional qualification in that member state are also open to nationals of the EU (and the other three states) that have gained a similar professional qualification in another member state.
Professions regulated in most or all EU states include teachers (Qualified Teacher Status in the UK), accountants (Chartered Accountant in the UK), and lawyers (barristers, solicitors and advocates in the UK). Note that the regulations only apply to nationals of the 18 countries - e.g. an American who gained Qualified Teacher Status in the UK would not be able, under these regulations, to teach in France, but an Irish citizen would.