  CMB-S4 is a proposed experiment to map the polarization of the Cosmic
Microwave Background (CMB) to nearly the cosmic variance limit for angular
scales that are accessible from the ground. The science goals and capabilities
of CMB-S4 in illuminating cosmic inflation, measuring the sum of neutrino
masses, searching for relativistic relics in the early universe, characterizing
dark energy and dark matter, and mapping the matter distribution in the
universe have been described in the CMB-S4 Science Book. This Technology Book
is a companion volume to the Science Book. The ambitious science goals of
CMB-S4, a "Stage-4" experiment, require a step forward in experimental
capability from the current Stage=II experiments. To guide this process, we
summarize the current state of CMB instrumentation technology, and identify R&D
efforts necessary to advance it for use in CMB-S4. The book focuses on
technical challenges in four broad areas: Telescope Design; Receiver Optics;
Focal-Plane Optical Coupling; and Focal-Plane Sensor and Readout.
