Every person seeks happiness by natural inclination, but not everyone seeks happiness in the same particular good. It is up to each individual to determine the particular good in which one seeks happiness. Cornelio Fabro calls this the existential choice - the choice of a concrete, existential, and determinate particular good in which happiness is sought. The existential choice guides each further action of an individual, and yet at the same time is either strengthened, weakened, or changed based on each concrete choice a person makes. Cornelio Fabro finds implicit support for the existential choice in Thomas Aquinas’s writings, and yet it is a new development of Aquinas’s thought. This thesis will examine the nature of the existential choice as a choice of end, the complex interaction between the intellect and the will in three realms (formal, metaphysical, and existential), as well as the morality and correctness of the existential choice. The existential choice is a determination of the particular good in which one seeks happiness, but it is not a guarantee of the attainment of happiness. Choices of finite goods will not lead to the full attainment of happiness, since it cannot fully be found in them. It is only through the choice of the Infinite Good, God, that happiness can truly be obtained. The connection between religious life and the existential choice of God will also be examined.