Blockchain is perceived as the foundation of a wide range of flexible ecosystems. People have more expectations from this paradigm than cryptocurrencies,, such as Bitcoin. However, most of all proofs of concepts and experiments are showing the gaps between the dream and reality, which are caused by the nature and design philosophy of blockchain technology. This book explains and analyzes the fundamental issue we face when we apply blockchain technology to real life applications. These issues are caused by not only a computer science point of view, due to the nature of technology design; the blockchain technology is an excellent mixture of mathematics, cryptography, incentive mechanism, economy and regulations. This book provides essential understanding regarding the reason why such issues occur by revising theories behind them. This book is useful to understand the theory of blockchain, the reason why blockchain is hard to adopt to real-life problems, but it still has value as a foundation for a flexible ecosystem, directions to solve these issues and find a suitable future area of blockchain applications. Authors of this book are experts from wide range of background like cryptography, distributed computing, computer science, trust, identity, regulation and standardization.