This introductory chapter surveys the evolution, application, and significance of momentum-control systems in spacecraft. It highlights the transition of these game-changing technologies from exclusive government use into the commercial sector. The chapter outlines the operational principles, advantages, and limitations of control-moment gyroscopes and reaction wheels compared to alternative attitude-control actuators like thrusters and magnetic torquers. It also traces the historical development of momentum-control from early spin-stabilized spacecraft to contemporary three-axis stabilized platforms, and it discusses emerging applications of CMGs in small spacecraft, satellite servicing, and asteroid missions. Finally, it frames the ongoing research challenges, especially in managing singularities within CMG arrays, setting the stage for deeper technical discussions in the subsequent chapters.