This dissertation develops an interpretation of the foundational commitments of Stoic ethics. I argue, first, that the Stoics are committed to rational eudaimonism, understood as the claim that all reasons for action are relative to and explained by an agent's own happiness. I argue, second, that this commitment clarifies the role of cosmic nature in Stoic theory and structures two fundamental Stoic doctrines, the doctrine of oikeiosis and the doctrine of preferred indifferents. According to the doctrine of oikeiosis, an organism's telos is realized through the perfection of its controlling faculty or hegemonikon. I argue that the Stoic account of self-perception is the most important element of this doctrine and helps to explain its role in Stoic ethical argument. According to the doctrine of preferred indifferents, although external circumstances make no difference to an agent's happiness, the rational Stoic agent will prefer some indifferent outcomes to others. I argue that, as a consequence of Stoic eudaimonism, the value of preferred and dispreferred indifferents should be understood as epistemic rather than intrinsic. I conclude by distinguishing the Stoic conception of practical reason from Humean and Kantian conceptions.