Abstract “Aggressive tax planning” (ATP) is typically characterized as a tax scheme that reduces the effective tax rate of a particular type of income to a level below the one sought by fiscal policy for this income. One motivation often suggested for its use is the uncertainty in tax liabilities introduced by a complicated and ever changing tax system. In this paper, I examine the impact of an uncertainty on the use of such tax schemes; by implication, I also examine how a simpler and more stable tax system that reduced this uncertainty might affect ATP. In this analysis, I draw upon some of my own work on tax avoidance and tax evasion, and then I extend this work to the related but separate area of ATP. Importantly, I introduce and model both individual and group motivations, incorporating insights from behavioral economics in these new analyses. Taxpayers are clearly motivated in part by narrowly defined financial considerations as shaped by the tax, audit, and penalty rates that they face, all of which I classify as individual motivations . However, individuals are also often influenced by many other factors that go beyond self-interest and that have as their main foundation some aspects of social norms, morality, altruism, fairness, or the like. In their entirety, I lump these factors together as group motivations , and I argue that they are shaped by the dynamic social context in which, and the process by which, decisions emerge. My main conclusion is that there is much in theory to suggest that uncertainty leads to more use of ATP, especially when both individual and group motivations are considered.