The chapter examines the legal challenges of rationality of automated decision-making through constitutional due process in the US, and via judicial review in the UK and Australia. The existing legal frameworks of these jurisdictions are premised on human decision-making and the concept of human rationality. Automated decisions that fail the test of rationality can be invalidated. Following this, the chapter will consider three main issues in terms of reviewability of the rationality of a decision: what is seen as constituting a “decision”, who is the decision-maker, and what factors and criteria can be used in making a decision.