The use of performance enhancing substances and methods (known as "doping") in sport is an intractable issue, with current anti-doping strategies predominantly focused on the personal responsibility and strict liability of individual athletes. This is despite an emerging understanding that athletes exist as part of a broader complex sports system that includes governance, policymakers, media, sponsors, clubs, team members, and athlete support staff, to name a few. As such, there is a need to examine the broader systemic factors that influence doping in sport. The aim of this systematic review was to identify and synthesise the factors contributing to doping and doping behaviours, attitudes, and beliefs and the extent to which this knowledge extends beyond the athlete to consider broader sports systems. The review followed PRISMA guidelines with risk of bias and study quality assessed by the Mixed Methods Appraisal Tool, and identified contributory factors synthesised and mapped onto a systems thinking-based framework. Overall, the included studies were determined to be of high quality. Support personnel, the coach, and the coach-athlete relationship represent key influences on the athletes' decisions to dope. From the evidence presented, doping is an emergent property of sport systems and represents a complex systemic problem that will require whole-of-system interventions. The implications for this and the focus of future research are discussed.