Macroscopic quantum phenomena are particularly important when considering the problem of Schrödinger's cat. This book contains a coherent and self-contained account of such phenomena, focusing on the central role played by macroscopic quantum tunneling. Beginning with an explanation of the nature and significance of the cat problem, Shin Takagi introduces the concept of macroscopic quantum tunneling. He deals with typical examples in detail, elucidating how quantum mechanical coherence may be lost (so-called 'decoherence') or how it may be maintained despite the effects of environment and measurement processes. Recent experimental and theoretical advances are discussed, and the remaining problems described. The final chapter describes an experiment to decide between quantum mechanics and macrorealism in the light of Einstein's moon. Assuming only a knowledge of elementary quantum mechanics, this book emphasises conceptual aspects rather than technical details. It provides a firm introduction to the subject for graduate students and researchers.