The ever-increasing presence of digital data and computers requires pushing for new levels of scalability and sustainability of computing systems in order to address the huge data and processing requirements of the near future. ultrascale computing systems (UCSs) are a solution. Envisioned as large-scale complex systems joining parallel and distributed computing systems, which can be located at multiple sites and cooperate to provide the required resources and performance to the users, they could extend individual systems to provide the resources needed. UCSs are expected to be two to three orders of magnitude larger than today systems, including systems with unprecedented amounts of heterogeneous hardware, lines of source code, numbers of users, and volumes of data. The book is composed of seven chapters covering aspects related to programming models, runtimes, resilience, data management, energy efficiency, and applications. Globally, the book shows that UCS development requires research and development of novel domain-specific, but interoperable tools to enable high productivity of human-computer interaction, leading toward robust solutions through multi-domain cooperative approaches using energy efficient hardware-software co-design principles.