Some embodiments provide a Quanta Image Sensor (QIS) comprising 3D vertically-stacked photosensor array and readout circuitry. In some embodiments, an imaging array comprises a plurality of single-bit or multi-bit jots, and readout circuitry in electrical communication with the imaging array and configured to quantize, for each jot, an analog signal corresponding to the electrical signal of the jot, wherein the imaging system is configured as a 3D vertically integrated circuit with the imaging array stacked vertically above the readout circuitry. The imaging array may be configured as an array of clusters with respect to the readout circuitry, each cluster configured as an array of n by m jots. The imaging array may include a further image processing circuitry layer disposed below the readout circuitry layer. Neighboring layers may be implemented on separate substrates and/or in a common substrate.