This book contains a coherent and self-contained account of macroscopic quantum phenomena, focusing on the central role played by macroscopic quantum tunneling. Beginning with an explanation of the nature and significance of the Schrödinger's cat concept, Shin Takagi introduces 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. Assuming only a knowledge of elementary quantum mechanics, this book emphasizes conceptual aspects rather than technical details.