Union royale belge des societes de football association v Jean-Marc Bosman

Judgment in the famous Bosman case by virtue of which it was considered that the article 48 of the EEC Treaty precludes the application of rules laid down by sporting associations, under which a professional footballer who is a national of one Member State may not, on the expiry of his contract with a club, be employed by a club of another Member State unless the latter club has paid to the former club a transfer, training or development fee.

For full case reference, here.