This thesis consists of four essays related to optimal antitrust enforcement. The
�rst essay provides a case study of EC ringleader cartels and discusses by means
of a theoretical model the e¤ect of excluding ringleaders from leniency programmes
on collusive prices. The second essay adds an experimental investigation of the
former, and examines in particular the e¤ects on cartel formation, prices and stability.
The third essay experimentally explores the substitutability of antitrust detection
rates and �nes, and test whether di¤erent �ne and detection rate combinations with
constant expected �nes achieve an equal level of deterrence. Lastly, the �nal essay
discusses the role of antitrust enforcement on collusion when �rms can engage in
avoidance activities
