Private Military Companies and Private Security Companies have come to play an increasingly more important role on the international stage today. The aim of this thesis is to investigate how these companies affect states’ monopoly on legitimate force, using the US as a case. The theoretical framework applied in this thesis formulates three hypotheses, which structure the investigation and make it possible to investigate central aspects of the monopoly thesis, as it has been formulated by Max Weber. Secondary analysis of formal studies is applied as methodology. The findings in this thesis suggest that Private Military Companies and Private Security Companies do affect states’ monopoly on legitimate force in a variety of ways.