This book investigates the relationship between cult and ethics in the book of Isaiah. Part I attempts to revise some of the common Old Testament views on prophets and cult. After inspecting cultic concepts such as sacrifice, purity and impurity, holiness, and the Promised Land, it suggests that the priestly and prophetic understandings of the role of the Ancient Israelite cult were essentially the same. This general proposition is then tested on the book of Isaiah in Part II: each chapter there analyses the key passage on cult and ethics in the three main parts of the book, namely, Isa 1:10-17; 43:22-28; and 58:1-14 and concludes that, even though the role of cult and ethics in each part of the book varies significantly, the underlying principles behind the teaching about ritual and social justice in the various parts of the book of Isaiah are the same. Furthermore, these principles are cultic in nature, and in accord with priestly teaching. Far from being anti-ritualistic, the studied texts are concerned with what can be labelled The Ethical Dimension of Cult. The reason behind the variations of the role of cult and ethics in the book called Isaiah seems to be cultic as well, namely the purity or impurity of the people and the land before, during, and after the Babylonian exile.