Notions such as complicity, accessory liability as well as aiding and abetting frequently appear in international criminal law. This study focuses on the relevant legal issues concerning secondary liability as a mode of individual criminal responsibility. Part A looks into the distinction between primary and secondary liability, the nature of secondary liability, and the approaches to prevent secondary liability from being overinclusive. Parts B to D systematically analyse the body of law that has been shaped by the trials conducted in the aftermath of the Second World War, has emerged from the ad hoc tribunals seized with the crimes committed in the former Yugoslavia (ICTY) and Rwanda (ICTR), and which has been created by the member states to the Rome Statute of the International Criminal Court (ICC). The main questions asked are how secondary liability relates to a system of perpetration and participation, how it is distinguished from other modes of liability and what its actus reus and mens rea elements are. In that regard, particular emphasis has been laid on the substantial and direct effect requirements, as well as on the notion of purposive facilitation.