Abstract: We describe the design, development, and demonstration of a frequency-domain multiplexed readout technology for superconducting transition-edge sensors (TESs). This technology enables us to deploy large-scale bolometer arrays for ultra-sensitive observations of the cosmic microwave background (CMB). Each sensor is biased with a sinusoidal voltage at a unique frequency. As the sensor impedance changes due to absorbed radiation, it amplitude-modulates its bias. The modulated sensor currents are summed and measured with a single superconducting quantum interference device (SQUID) array. The 100-element SQUID array is operated with custom shunt feedback electronics that have a bandwidth of 1MHz and a slew rate of 1.2 × 107Φ ₀/s. Separate warm demodulation electronics recover the sky signal from each sensor. Our eight-channel demonstration of readout multiplexing showed a multiplexer readout noise of < 7 pA/$Hz$ (as referred to the input of the SQUID), well below the sensor noise of 15 pA/$Hz$. Crosstalk between multiplexed channels is well below our design limit of 1% and demodulated noise spectra from multiplexed sensors show the expected sensor noise levels and are white at frequencies down to 200 mHz. The current frequency-domain multiplexing technology will be used on several experiments designed to observe the CMB: APEX-SZ, the South Pole Telescope, Polarbear, and EBEX. Extensions to the technology are described that will enhance the performance of the system and allow multiplexing of ∼ 10 2 detectors.