Megaprojects are large and are constantly growing ever larger, and more and more are being built in what has been called the biggest investment boom in history. This chapter serves as an introduction to megaprojects in which they are defined and in which the size of global megaprojects business is estimated. Drivers of the megaproject boom are identified, including monumentalism and the technological sublime; ten things you must know about megaprojects are detailed, from their tendency to suffer from uniqueness bias to their overexposure to black-swan events; and the “iron law of megaprojects” is identified as a main challenge to megaproject management: “Over budget, over time, under benefits, over and over again.” Finally, the main structure of the Handbook is set out as covering the what, the why, and the how of megaproject management, in terms of the challenges, causes, and cures that students of megaprojects must decipher to better understand and better manage them.