One of the most significant trends in the evolution of global tax systems has been the rise from relative obscurity of thin capitalisation rules, which are perceived as anti-avoidance rules which limit tax base erosion from cross-border interest deductions. However, over the same timeframe, innovations to financial instruments have challenged the traditional financial and legal distinctions between debt and equity, which in the cross-border setting has exposed the prevalence of economic inefficiencies in the design of the international tax system.