  Single-photon avalanche diode (SPAD) arrays are solid-state detectors
offering imaging capabilities at the level of individual photons, with
unparalleled photon counting and time-resolved performance. This fascinating
technology has progressed at very high pace in the past 15~years, since its
inception in standard CMOS technology in 2003. A host of architectures has been
explored, ranging from simpler implementations, based solely on off-chip data
processing, to progressively ``smarter" sensors including on-chip, or even
pixel-level, timestamping and processing capabilities. As the technology
matured, a range of biophotonics applications has been explored, including
(endoscopic) FLIM, (multi-beam multiphoton) FLIM-FRET, SPIM-FCS,
super-resolution microscopy, time-resolved Raman, NIROT, and PET. We will
review some representative sensors and their corresponding applications,
including the most relevant challenges faced by chip designers and end-users.
Finally, we will provide an outlook on the future of this fascinating
technology.
