The CMB-S4 experiment is the next generation ground-based cosmic microwave background (CMB) project, designed to make a dramatic leap forward in understanding the fundamental nature of space and time and the evolution of the Universe. CMB-S4 will use highly sensitive superconducting cameras at the South Pole, the high Chilean Atacama plateau, and possibly northern hemisphere sites to cross critical thresholds in testing inflation, determining the number and masses of neutrinos, constraining possible new light relic particles, providing precise constraints on dark energy, and testing general relativity on large scales. CMB-S4 is intended to be the definitive ground-based CMB project, delivering a highly constraining dataset that will allow any model for the origin of primordial fluctuations to be consistent with observations. It will also maximize the discovery space from CMB polarization, lensing, and other secondary effects by producing high-fidelity maps. CMB-S4 is the logical successor to the Stage-3 CMB projects and should be implemented on a schedule that allows a seamless transition from Stage 3 to Stage 4. It will also complement ongoing optical surveys like LSST, DESI, Euclid, and WFIRST. CMB-S4 will use proven existing technology, which has been developed and demonstrated over the last decade, and will require scaled-up superconducting detector arrays, high-throughput mmwave telescopes, and computational methods for extracting correlations in massive data sets. CMB-S4 is well aligned with the Particle Physics Project Prioritization Panel (P5) report, which recommends supporting CMB experiments as part of the core particle physics program. The P5 report also endorses CMB-S4 as one of three priority research initiatives in the NRC report. CMB-S4 will make definitive CMB measurements at angular scales from tens of degrees to arc minutes, with a unique synergy with satellite-based measurements. It will provide insights into inflationary gravitational waves, dark energy, dark matter, and light relics. CMB-S4 will also map all the mass in the Universe through gravitational lensing, and will improve the precision of measurements of the total energy density in light weakly-coupled particles. CMB-S4 will also provide constraints on the parameters of dark matter and dark energy, and will test general relativity on large scales. The CMB-S4 Science Book sets the scientific goals for CMB-S4 and the measurements required to achieve them. It provides the basis for proceeding with the detailed experimental design. The Science Book includes summaries of the primary science drivers, observables, and analysis/computing issues, each of which is developed in depth in a dedicated chapter of the book.The CMB-S4 experiment is the next generation ground-based cosmic microwave background (CMB) project, designed to make a dramatic leap forward in understanding the fundamental nature of space and time and the evolution of the Universe. CMB-S4 will use highly sensitive superconducting cameras at the South Pole, the high Chilean Atacama plateau, and possibly northern hemisphere sites to cross critical thresholds in testing inflation, determining the number and masses of neutrinos, constraining possible new light relic particles, providing precise constraints on dark energy, and testing general relativity on large scales. CMB-S4 is intended to be the definitive ground-based CMB project, delivering a highly constraining dataset that will allow any model for the origin of primordial fluctuations to be consistent with observations. It will also maximize the discovery space from CMB polarization, lensing, and other secondary effects by producing high-fidelity maps. CMB-S4 is the logical successor to the Stage-3 CMB projects and should be implemented on a schedule that allows a seamless transition from Stage 3 to Stage 4. It will also complement ongoing optical surveys like LSST, DESI, Euclid, and WFIRST. CMB-S4 will use proven existing technology, which has been developed and demonstrated over the last decade, and will require scaled-up superconducting detector arrays, high-throughput mmwave telescopes, and computational methods for extracting correlations in massive data sets. CMB-S4 is well aligned with the Particle Physics Project Prioritization Panel (P5) report, which recommends supporting CMB experiments as part of the core particle physics program. The P5 report also endorses CMB-S4 as one of three priority research initiatives in the NRC report. CMB-S4 will make definitive CMB measurements at angular scales from tens of degrees to arc minutes, with a unique synergy with satellite-based measurements. It will provide insights into inflationary gravitational waves, dark energy, dark matter, and light relics. CMB-S4 will also map all the mass in the Universe through gravitational lensing, and will improve the precision of measurements of the total energy density in light weakly-coupled particles. CMB-S4 will also provide constraints on the parameters of dark matter and dark energy, and will test general relativity on large scales. The CMB-S4 Science Book sets the scientific goals for CMB-S4 and the measurements required to achieve them. It provides the basis for proceeding with the detailed experimental design. The Science Book includes summaries of the primary science drivers, observables, and analysis/computing issues, each of which is developed in depth in a dedicated chapter of the book.