This chapter, An Age of Megaprojects, presents the case for the book, its aims, analytical structure, methodology, and the chapter summaries. In the book, megaprojects and their impacts are empirically examined, conceptually framed, and positioned vis-à-vis urban sustainability. We show the resemblances among the projects we analyze, despite the very different political, social, geographic, and developmental contexts in which they bloomed within the past 40 years. We link the development and impact of megaprojects to key causal forces, relevant governance systems, and related developmental discourses. Megaprojects contribute to urbanization and, consequently, to the resultant sustainability challenges, both socioeconomic and environmental. A key question, then, is whether the diverse and negative impacts of megaprojects are compatible with any approach to urban sustainability that prioritizes socioeconomic equity and environmental preservation. Our analytical lens of disruptive complexity provides insights that most critiques of megaprojects do not include. Thus, we show that cost overruns and poor management, although both are common, are not the only or the worst effects of megaprojects; rather the primary reason to criticize megaprojects is their disruptive impacts on the cities where they are built—socioeconomic, environmental, spatial, political and geopolitical, financial, cultural, and systemic impacts.