The aerospace industry demands an incredible level of precision to ensure the risk of disaster is minimized. Multiple sensing systems are needed to provide an accurate overall assessment and facilitate decision-making. Thus, this introductory chapter aims to provide an overview of optical sensors and their crucial role in modern aerospace applications. It begins by introducing some of the key specific requirements and challenges in aerospace environments (namely extreme temperatures and radiation exposure), and provides some insights on the advantages of optical over traditional electronic-based sensing. This is followed by a brief outline of some fundamental areas where optical sensors can be applied to aerospace applications, namely in navigation, structural health monitoring, environmental sensing, among others. This chapter introduces the scope of this book, while paving the way for a more comprehensive and focused discussion in subsequent chapters.