The book "Moral Theology. A Complete Course Based on St. Thomas Aquinas and the Best Modern Authorities" by Charles J. Callan and John A. McHugh is a study and systematization of moral theology based on the ideas and principles of St. Thomas Aquinas and the best modern authors in this field . The book reveals and analyzes the main concepts of morality, moral action, responsibility, conscience, good and evil, loyalty, justice and other aspects that form the basis of Christian morality. The authors, through a comprehensive approach to the topic, try to give the reader a deep understanding of the essence and meaning of moral principles within the framework of the modern world. This book will be an important source of knowledge for theologians, philosophers, students, and anyone interested in moral theology and contemporary research in this field.