Council Regulation (EC) 1/2003 came into force in May 2004 and replaced the mandatory notification of agreements by a regime of ex post monitoring. This paper shows that ex post monitoring is the optimal audit regime when the competition authority's probability of error is low. On the other hand, the notification system becomes optimal when the competition authority's probability of error is high. The paper also shows that block exemptions and black list (regimes of per se (il)legality) should be implemented when the agreements' expected welfare impact is very high and very low, respectively.