The topic of conscience has fascinated me for a number of years. In the course of my pastoral work in different parishes, and chaplaincy work with young people and those in hospital, whether for mental or physical reasons, I have encountered countless people who struggled with moral dilemmas or who felt crushed by guilt owing to the gap between their practice and their knowledge of what they were called to do. I have also met individuals who seemed to be unaware that, despite the gravity of the action, what they were doing was in any way wrong. Whether mentioned or left implicit, the conscience of each of these individuals played a vital role in the decision to choose one course of action over another, in judging a completed action to have been right or wrong, or even in exhibiting a state of perplexed uncertainty as to what should be done next. Thinking over these different problems led me to reflect further on the question of erroneous conscience. Could an action that was considered to be wrong by others (particularly by the Magisterium of the Church) be good, virtuous or meritorious if the individual believed it to be so? Do we live in parallel moral universes, where the person ultimately defines what is moral solely by belief or conviction, or do we have access to a ground of universal truth, rooted in our created nature, as gifted by God? Therefore, should the pastor leave the individual in blissful ignorance, or should he try to deepen moral understanding or help develop the moral capacities of the people he encounters? Consideration of questions such as these led me to investigate the possibility of making conscience the subject of doctoral research. However, the resultant thesis is not a study of pastoral problems, in the style of a manualistic analysis of cases of conscience. Rather the study is at the level of fundamental moral theology, presented in the hope that a deepened awareness of the nature and function of conscience will shape my future pastoral activity, and in the hope that it might help others, too, in their understanding of this core notion of morality.