Taxation of cross-border investment activity is inherently more complex than tax compliance relating to purely domestic activity, because of the interaction of multiple tax regimes and tax administrations. That inherent complexity can be mitigated by effective multilateral co-operation, which can reduce the challenges that otherwise arise from having multiple sets of uncoordinated rules. Seeking greater simplification through multilateral co-operation efforts can generate a range of benefits by fostering greater tax certainty, which can in turn generate growth. This report explores the key aspects of simplicity, the drivers of complexity, and the trade-offs involved, looking both at the domestic context and the context of multilateral co-operation. Various factors are examined to help guide both the policy and rule design process and the development of substantive solutions in multilateral co-operation efforts, highlighting opportunities to reduce unnecessary complexity. The report also considers next steps, including broader consultation and identifying possible priority areas for further work on simplification in cross-border business taxation. Future work could be undertaken with other interested jurisdictions, organisations and stakeholders, to build towards a practical work plan for simplification. This report was prepared at the request of the South African G20 Presidency.