Today, cyberspace has emerged as a domain of its own, in many ways like land, sea and air. Even if a nation is small in land area, low in GDP per capita, low in resources, less important in geopolitics, low in strength of armed forces, it can become a military super power if it is capable of launching a cyber-attack on critical infrastructures of any other nation including superpowers and crumble that nation. In fact cyber space redefining our security assumptions and defense strategies. This book explains the current cyber threat landscape and discusses the strategies being used by governments and corporate sectors to protect Critical Infrastructure (CI) against these threats.