This chapter provides a comprehensive exploration of the foundational principles and transformative potential of brain-computer interface (BCI) technology. We begin with an introduction to the fundamental processes by which brain signals are encoded and decoded to control external electronic or mechanical devices. This core mechanism forms the backbone of any BCI system. Through various scenarios and detailed examples, the chapter highlights the vast and diverse applications of BCIs, ranging from medical rehabilitation and communication aids to nonmedical uses in entertainment and security. This chapter serves as the foundation for all the chapters in this book.