This thesis discusses the theory, architecture design, circuit design and measurements of an ultra-low-energy reconfigurable interface circuit for resonant gas sensors. This interface circuit employs a transient measurement method. The resonant sensor is driven at a frequency close to its resonance frequency by an excitation source that is intermittently disconnected, causing the sensor to oscillate at its resonance frequency with an exponentially-decaying amplitude. From the associated ring-down signal, the frequency of the freely-oscillating sensor and its quality factor are obtained by means of a counting technique. A prototype readout circuit that senses the ring-down signal and performs the required level-crossing detection has been fabricated in a standard 0.35-?m CMOS technology. The experimental results obtained using this prototype in combination with samples of micro-machined clamped-clamped beam resonators show good consistency with the resonance frequency and quality factor obtained using conventional impedance analysis. Compared to prior implementations, the realized prototype is less sensitive to leakage currents, enabling a shorter measurement time, and provides a reconfigurable front-end circuit that allows it to be connected to resonators with different parameters. The circuit consumes an energy of 207.9nJ per measurement.