Spontaneity is a pervasive feature of our mental lives. Many of our thoughts and actions seem to unfold in the absence of explicit, cognitive control. This has led some to argue that spontaneous cognitive processes are always experienced passively: spontaneous cognition is the sort of thinking that seems to just happen to us. That is, it doesn’t usually appear as if we are in control of the dreams we have at night, the drifting of our attention as our minds wander, the ruminations we have about our concerns, or the flashes of creative insight we might have as we go about our life projects. When we experience these sorts of cognitive processes, they often seem to happen to us in the absence of any deliberation or effort. Instead of willfully generating the thoughts that inhabit our minds in such cases, these thoughts seem to spontaneously strike us.This dissertation argues that spontaneity in cognition and action does not necessarily imply passivity. Although spontaneous thoughts and actions may be experienced passively in many cases, I think there is also a sense in which we can let spontaneous thoughts and actions unfold. For example, one can intentionally let one’s mind wander during an inherently boring situation such as sitting in standstill traffic. Similarly, one may intentionally engage in an episode of reflective rumination, repetitively engaging the same ideas to solve a problem. In the realm of spontaneous skillful action, expert improvisers often claim they perform best when they intentionally let the actions flow through them. And in habitual behavior, such as habitual smartphone use, one might intentionally let their absent-minded scrolling behavior continue. In all cases, I think intentionally letting spontaneous processes unfold qualifies as a sort of mental action, in contrast to those who think that spontaneity implies passivity. At first glance, this may seem like a contradiction: intentional spontaneity seems oxymoronic. This dissertation will argue that this is not the case. I think one can intentionally engage in spontaneous thinking and acting via metacontrol, a form of metacognition that is directed at other control processes. In this dissertation, I will argue metacontrol underlies individuals’ ability to intentionally engage in spontaneous cognition and action. In particular, I characterize metacontrol in the context of intentional mind-wandering, rumination, lucid dreaming, improvisation, and doomscrolling to illustrate how metacontrol influences a range of both adaptative and maladaptive patterns of thinking and behavior.