This book introduces the UTCI (Universal Thermal Climate Index) and summarizes progress in this area. The UTCI was developed as part of the European COST Action Program and first announced to the scientific community in 2009. Since then, a decade has followed of applicability tests and research results, as well as knowledge gained from applying the UTCI in human adaptation and thermal perception. These findings are of interest to researchers in the interdisciplinary areas of biometeorology, climatology and urban planning. The book summarizes this progress, discussing the limitations found and provides pointers to future developments. It also discusses UTCI applications in the areas of human biometeorology and urban planning including possibilities of using UTCI and similar indices in climate-responsive urban planning. The book’s message is illustrated with many case studies from the real world. Chapter 10 is available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.