Abstract: Classical theories of epistemic rationality take an agent's individual beliefs to be the only things that are rational or irrational. For them, rationality is wholly static. Recent work in epistemology take sets of individual beliefs and also changes of belief over time to be rational or irrational. For these theories, rationality is both static and dynamic. However, for both groups, static rationality is fundamental. In my dissertation, I argue to the contrary that, in fact, all rationality is dynamic rationality. Epistemic reasons, rationality, and justification as applying only to changes of belief. This wholly dynamic view of rationality, which I call 'Dynamicism' has wide-ranging epistemological consequences. A small set of simple, elegant, and independently motivated principles of dynamic rationality can illuminate and solve otherwise interminable epistemological disputes. Chapter One refutes the view that dynamic rationality can be reduced to static rationality. Chapters Two and Three extend the arguments from Chapter One to argue for skepticism about the concept of static rationality altogether. Chapter Four adopts Dynamicism as a working hypothesis and presents a dynamic proto-theory of rationality for all-or-nothing beliefs. Chapter Five turns to the philosophical upshots of Dynamicism for some problems of epistemology. Of central importance is the way in which Dynamicism reformulates a variety of issues centered around the problem of skepticism, including the dialectic between skepticism and dogmatism, closure principles for justification, and Conservatism.