In common with other philosophers of existence, Gabriel Marcel is deeply concerned about life as it affects the individual in his situation-in-the-world. Marcel's uniqueness stems from his attitude to the meaning of life. His whole philosophy can be summed up as the expression of an option: that life can have a positive meaning. This can be appreciated, he asserts, if life is seen in a theocentric perspective which is itself possible only when the "sacral" character of life is recognized. Marcel suggests that this recognition will lead to the awareness by man of his situation as a being participating in the overall Mystery of Being. The basis of his metaphysics is an ontology of participation on three interconnective levels: of sensation (the self), of communion (others), and of transcendence (union with God). While he insists on the individual's freedom to accept or reject this option, Marcel's "concrete" philosophy of existence is revealed as an "applied theocentrism". This theo-centric orientation is at the heart of his three "concrete" approaches to the mystery of being - fidelity, hope and love - which, because they are directed towards and grounded in God, are shown to be philosophical translations of the three "theological" virtues - faith, hope and charity. According to Marcel's dialectic, man's consideration of self leads to the awareness of his ontological status as creature dependent on God as Creator. In his relations with others the individual is led to acknowledge that the ground and guarantee of all I-thou relationships is God as Absolute Thou. Finally, man's aspiration to transcendence is sublimated in union with God; this possible union involves the question of survival after death, which Marcel postulates in the immortality of fidelity in the sustained presence of those who have loved and who have been loved. The conclusion reached is that Marcel’s dialectic is an attempt at an "applied theocentrism" so that not only is God central in Marcel's thought but Being is identifiable with God, in the manner in which Marcel understands these terms. The Mystery of Being is seen as the ontological counterpart of the Mystery of God. The "need" (exigence) for being is translated, in the terms of Marcel's metaphysics, into the need for God. The fundamental option on the ontological plane, to be or not to be, is to be understood as the counterpart of the real fundamental option in Marcel's dialectic of the meaning of life, to believe or not to believe in God. Throughout the thesis, certain deficiencies in Marcel's writings are pointed out. These include an excessive religious element, traces of literary dilettantism and elitism, the aphoristic nature of many of his pronouncements, his reluctance to give rational grounds for these statements, his evasiveness in that he glosses too readily over awkward questions and avoids giving any firm indication of his definitive position. These deficiencies outweigh his more positive contributions to a philosophy of existence, albeit a Christian philosophy of existence, so that his propaedeutics to a metaphysics of being remains a propaedeutics.