The CMB-S4 collaboration aims to conduct the next-generation ground-based cosmic microwave background (CMB) experiment, which will provide significant advancements in understanding the fundamental nature of space and time and the evolution of the Universe. CMB-S4 will be equipped with dedicated telescopes at the South Pole, the high Chilean Atacama plateau, and possibly northern hemisphere sites, using highly sensitive superconducting cameras. The primary scientific goals of CMB-S4 include testing inflation, determining the number and masses of neutrinos, constraining new light relic particles, providing precise constraints on dark energy, and testing general relativity on large scales. CMB-S4 is designed to be the definitive ground-based CMB project, delivering a highly constraining dataset that will test various models of the early Universe and their evolution. The collaboration emphasizes the importance of high-fidelity maps and the legacy value of CMB-S4 data. The project is aligned with the Particle Physics Project Prioritization Panel (P5) report and endorsed by the National Research Council (NRC) report on NSF investments in Antarctic and Southern Ocean research. The scientific goals of CMB-S4 are detailed in several chapters, covering topics such as inflation, neutrinos, light relics, dark matter, dark energy, and CMB lensing. The book also discusses the challenges and technical considerations in data analysis, simulations, and forecasting. The collaboration has engaged a global community of scientists to contribute to the development and implementation of CMB-S4, ensuring a seamless transition from Stage 3 to Stage 4 experiments.The CMB-S4 collaboration aims to conduct the next-generation ground-based cosmic microwave background (CMB) experiment, which will provide significant advancements in understanding the fundamental nature of space and time and the evolution of the Universe. CMB-S4 will be equipped with dedicated telescopes at the South Pole, the high Chilean Atacama plateau, and possibly northern hemisphere sites, using highly sensitive superconducting cameras. The primary scientific goals of CMB-S4 include testing inflation, determining the number and masses of neutrinos, constraining new light relic particles, providing precise constraints on dark energy, and testing general relativity on large scales. CMB-S4 is designed to be the definitive ground-based CMB project, delivering a highly constraining dataset that will test various models of the early Universe and their evolution. The collaboration emphasizes the importance of high-fidelity maps and the legacy value of CMB-S4 data. The project is aligned with the Particle Physics Project Prioritization Panel (P5) report and endorsed by the National Research Council (NRC) report on NSF investments in Antarctic and Southern Ocean research. The scientific goals of CMB-S4 are detailed in several chapters, covering topics such as inflation, neutrinos, light relics, dark matter, dark energy, and CMB lensing. The book also discusses the challenges and technical considerations in data analysis, simulations, and forecasting. The collaboration has engaged a global community of scientists to contribute to the development and implementation of CMB-S4, ensuring a seamless transition from Stage 3 to Stage 4 experiments.