The Turkic Republics are increasingly recognized as key contributors to global development. Endowed with abundant natural resources and strategically located along major trade corridors, they have recently united under the Organization of Turkic States (OTS) to harmonize policies and strengthen regional cooperation. Azerbaijan, Kazakhstan, Kyrgyzstan, Türkiye, Uzbekistan, and the Turkish Republic of Northern Cyprus face a shared challenge: the prevalence of large informal economies. This informality undermines transparency, weakens governance, and impedes sustainable growth. This volume provides the first systematic comparative analysis of shadow economies across the Turkic Republics. Seven country chapters and an introductory comparative chapter examine the causes, consequences, and policy approaches to informality in the OTS. The book highlights both shared legacies and diverging institutional paths. It contrasts the socialist legacies of the post-Soviet republics with the market-oriented traditions of others in the region, illustrating how these distinct backgrounds shape shadow economies. It also documents the persistence of informal activity and its intersections to institutional trust, labor markets, digitalization, and sustainability. By combining cross-country comparisons with detailed national perspectives, the volume advances scholarship and offers actionable insights for policymakers aiming to reduce shadow economies and foster sustainable development.