We are developing a new class of elementary particle detectors capable of sensing weakly interacting particles such as neutrinos. Our detectors are based on the propagation of phonons in silicon crystals at low temperatures. During the past several years, we have made substantial progress towards the detailed understanding of phonon-mediated particle detection using silicon crystals with superconducting transition-edge sensors [B. A. Young, et al, Nucl. Instr. Meth. A311 195 (1992)]. We call these devices silicon crystal acoustic detectors or SiCADs. The transition-edge sensors are biased in temperature below their superconducting transition and in current below the latching current, the boundary between self-extinguishing pulses and thermal runaway from I2R heating. In this mode, the phonons from a particle interaction in the silicon crystal heat regions of the transition-edge sensors, driving those regions normal, and producing a voltage across the device that is proportional to the length of the normal region. In turn, the voltage defines a unique deposition energy once the depth of the event is determined from timing information.