In this chapter, McKinlay argues that the use of big data algorithms introduces a key problem in terms of epistemic opacity. Opacity in various forms is an issue which many authors identify as posing problems for democratic functioning and accountability. In McKinlay’s case, the argument focuses on the impact that epistemic opacity has on our ability to trust non-human agents. He holds that while the outputs of big data-derived decisions can be significant for citizens, where we do not have the ability to understand how these decisions were made, we cannot ultimately trust the decider. Decisions based on mere probability are not, he argues, sufficiently grounded for democratic systems and risk harming citizens.