We describe the present state of the art of time-domain SQUID multiplexers to read out large arrays of superconducting transition-edge sensor (TES) bolometers. Time-division SQUID multiplexers are used in a large range of millimeter-and submillimeter-wavelength astronomical instruments, including ACT, SPIDER, BICEP-2, SPUD, Clover, and SCUBA-2. They are also being developed for use in the Constellation-X X-ray observatory. We describe the advantages and disadvantages of time-division multiplexers, and consider the technological readiness level for a CMB polarimetry satellite mission. We describe both the cryogenic components and the present-generation warm readout electronics.