In this thesis I will examine St. Thomas Aquinas’s treatment of human nature and connect my findings with his conception of the ultimate end o f human existence. As a theologian St. Thomas held the position that man’s ultimate goal is happiness or beatitudo - which consists in the vision of God. Thomas explores the resources that are to be found in human nature and in particular those that are needed in order to achieve happiness to some degree in this life, and then considers the infinite happiness that is to be found in God alone. I show how St. Thomas’s solution to the mind-body problem is relevant today, albeit in a world which measures success in terms o f power and wealth but yet longs for what today we might term a spiritual dimension to our lives. The underlying principle for St. Thomas is that the rational soul is the unique form of the body, that is, that the soul actualises the body. Body and soul form a composite, a unity o f matter and form. In exploring the powers of the soul and St. Thomas’s explanation for the soul’s immortality I examine some of the interpretations made by contemporary Thomist scholars. St. Thomas emphasises the autonomy of the person, the capacity to reason and to make choices. In order to explore St. Thomas’s ethics I will consider intellect and will which are, for him, the two great powers of the soul and show that although distinct they are not separable. St. Thomas believes that we can and do act with real freedom, otherwise we cannot speak meaningfully about responsibility and in addition the application of reward and punishment would be futile. The second part of my thesis is a discussion of St. Thomas’s treatise on happiness, highlighting his conviction that human beings are not free in one respect - that is in their desire for the certainty of eternal life or beatitudo. According to St. Thomas human beings always act according to what we believe to be the ‘good’ but being human also means that we can be very much mistaken in our judgements and decisions. Finally I aim to show that St. Thomas’s overall achievement was to produce a synthesis of Christian philosophy with the natural philosophy o f Aristotle. Also, by drawing on elements from Jewish and Islamic thought, St. Thomas proves that it is possible for us in the twenty first century to move forward and to explore every avenue to find a common ground between the various disciplines of science, philosophy and theology. All have the common goal of seeking to understand and explore human nature and human destiny.