This opening chapter will explore the historical significance and development of direction-finding (DF) systems. We will explain the various applications of DF across different sectors of society and provide estimates of their usage. Additionally, we will offer a brief overview of the algorithms [1] used in DF systems and discuss their evolution. Ultimately, we will give a glimpse into DF for wideband signals, with more detailed information reserved for future chapters. We will introduce the fundamental principles of DF systems to ensure that readers are well-prepared for the chapters that follow. Furthermore, we will examine the impact of high-speed computing and the advancements in high-speed analog-to-digital converters. Finally, we will discuss the transition from the analog to the digital domain and its influence on the field of direction finding.