This book presents the state of the art in High-Performance Computing on modern supercomputer architectures. It addresses trends in hardware and software development in general. The contributions cover a broad range of topics, from performance evaluations in context with power efficiency to Computational Fluid Dynamics and High-Performance Data Analytics. In addition, they explore new topics like the use of High-Performance Computers in the field of Artificial Intelligence and Machine Learning. All contributions are based on selected papers presented in 2021 at the 31st Workshop on Sustained Simulation Performance, WSSP31, held at HLRS in Stuttgart, Germany, and WSSP32, held at Tohoku University in Sendai, Japan.