As with previous editions of this title, our principal aim is to present a coherent account of the Earth that will satisfy advanced students with diverse backgrounds. We have endeavoured to explore the physical principles of the subject in a way that encourages critical appraisal. This requires the reader to have some familiarity with a wide range of inter-related ideas, for which there is no clearly preferred, logical order of presentation. Should the properties of meteorites precede or follow the isotopic methods used to study them? Is it important to understand something about the Earth's internal heat before studying seismology or vice versa? Can we be clear about the evidence for tectonic activity without knowing about the behaviour of the geomagnetic field? We have attempted to avoid the need for answers to these questions by beginning each chapter with what we call a preamble. Our preambles are not intended to be synopses of the chapters or even introductions in the conventional sense, but glue to hold the subject together, with glimpses of related concepts from other chapters. We hope to convey in this way a feel for the unity of the subject. Especially for students using this book as a text, we suggest reading all of the preambles before looking deeper into any of the chapters.