Ambush marketing is a tactic used by companies to promote themselves at major events without being an official sponsor. It involves associating a brand with an event through advertising or other means. Some examples include Nike sponsoring athletes at the Olympics when Reebok was the official sponsor. Ambush marketing can negatively impact events by decreasing the commercial value for official sponsors. Organizers try to counter it through controlling advertising, enforcing rules on players, and passing strict anti-ambush laws.