their application in the domain of cryptography is very broad. A fundamental question regarding zero-knowledge protocols is whether their security is maintained when many examples of protocols run simultaneously. This question is also interesting from a theoretical point of view - in addition to challenges from an algorithmic perspective, the study of zero-knowledge in concurrent setting has contributed greatly to our understanding of the security of cryptographic protocols simultaneously. Protocols that remain zero-knowledge when many instances run simultaneously at the same time called zero-knowledge, and this book is devoted to studying them. This book presents the construction of zero-knowledge protocols simultaneously, along with proof of security.