Under the title of 'participation', theologians and philosophers have explored what it means for the reality that we observe, and in which we dwell, to have its origin in a divine or transcendent source. This introductory chapter surveys what is to come in this book on the theme of participation. It considers the principal sources that are to be used, especially the Christian Bible and the writings of Thomas Aquinas. It also considers the relation between philosophy and theology, and between conceptual studies and their application within the practical details of a life lived with an eye to participation.