This chapter opens the book by introducing the characteristics and particularities of critical infrastructures. Their existence and interplay forms a vital pillar of contemporary societies, and their protection is a top duty of governments and security research. Recent years have shown a paradigm shift of cyber-attacks from specific individual threat and attack scenarios, to a modern combination of various attack types and strategies to what we call an advanced persistent threat (APT) today. This term describes a diverse class of attacks that all share a set of common characteristics, which presents new challenges to security that demand urgent and continuous action by practitioners, researchers and every stakeholder of a critical infrastructure. The main focus of the book is describing game theory as a tool to establish security against APTs, and to this end, the introduction here starts with the abstract characteristics of an APT, showcasing them with a set of selected real-life documented cases of APTs that ends the chapter.