This dissertation focuses on several questions. How should we demarcate our concepts of cognitive endorsements (where “cognitive endorsements” include belief, acceptance, supposition, presupposition, and hypothesizing)? Are cognitive endorsements inherently normative states? Finally, how do cognitive endorsements factor into rationality and understanding?In addressing these questions, I argue for several theses. I first show that belief, acceptance, supposition, presupposition, and hypothesizing are unique cognitive endorsements with distinct features concerning (i) their conceptual links to the truth, (ii) their rational basis, and (iii) their connection to an agent’s will. Second, I argue that consciously formed cognitive endorsements engender distinct cognitive commitments that normatively constrain how one ought to treat the target propositions of these states. Third, I defend the view that diachronic belief-formation is governed by semantic commitments. Semantic commitments are normative constraints to form new beliefs depending on one’s current beliefs, cognitive abilities, conceptual resources, and the logical relations that hold between propositions and the contents of one’s current beliefs. Fourth, I argue that these commitments factor into rationality. More precisely, I defend a view called, “Robust Rationality.” On this view, epistemic rationality consists of external, mind-independent rules and internal, mind-dependent doxastic commitments that govern beliefs. Finally, I argue that rational cognitive endorsements can facilitate cognitive achievements of understanding. Understanding, I contend, comes in different species: veridical understanding, conjectural understanding, and narrative understanding. The type of understanding an agent possesses will depend on the underlying cognitive endorsement and the degree and quality of the relevant rational support.This project has important consequences for epistemology and philosophy of mind. It establishes a taxonomy of cognitive endorsements and their conditions of rationality. This taxonomy can be employed for theoretical work mapping the plurality of ways rational agents can endorse and employ propositions in thought. Second, it is shown that both acceptance and supposition play an important role in manifesting rational agency and shaping our epistemic lives. Third, this project illuminates the normative structure of our thinking, reasoning, and processes of inquiry. For it shows that our cognitive endorsements are inherently normative states. Furthermore, this project advances our understanding of epistemic rationality and understanding.