In the circuit model for SNSPD operation developed in the early works on these devices,11–13 the detector is represented as a variable resistor (hotspot) in series with an inductor (dominated by the kinetic inductance), and the readout circuit is viewed as a simple, dc-coupled impedance to ground. This model results in the simple detector dynamics described in 11, where the electrical behavior of the detector is purely inductive outside of the very short period when the wire has an electrical resistance and Joule heating plays a role. However, as described in Refs. 14 and 15, in nearly all SNSPD systems to date, the load seen by the detector is not simply resistive and independent of frequency as in this simple model, resulting in an effective nonlinear feedback, which couples the average count rate with the DE and, at extremely high count rates, can even drive the detector into the latched state (where it becomes insensitive to photons and must be reset by turning the bias current down externally).15 To avoid this nonlinearity, one must operate at very low count rates of order times slower than or below (where is the inductance-limited reset time of the device) or employ a readout approach that alleviates this undesirable feedback. Alternatively, higher count rates can be achieved using multielement SNSPDs,16 which can also enable limited photon-number resolution and single-photon imaging. One of the big challenges to realize the multielement SNSPDs is readout electronics. As the increased number of readout cables causes a significant heat load to the cryocooler and increased complexity, it is highly desirable to use cryogenic signal processing to alleviate the need for a separate cable for each detector element. In this section, approaches for removing the count-rate-dependent feedback and electrically addressing multiple, independent detector elements will both be described.