The principal-agent theory from economics is applied to public procurement law in order to minimize opportunism risks in large-scale IT projects. The digital transformation will make it unthinkable to provide services of general interest without IT systems. In large-scale IT projects in particular, there is a risk that highly specialized IT companies will unilaterally exploit knowledge deficits on the part of the public sector customer and lock-in effects to their own advantage. The new legal concept of "Digital Services of General Interest 4.0" is intended to contain this risk. The study is addressed to contracting authorities, public IT service providers and the research community.