SPADs (Single-Photon Avalanche Diodes) are the viable photodetectors for most single-photon counting and photontiming applications. Some custom SPAD and many complementary metal-oxide semiconductor (CMOS) SPADs have been reported in literature, with quite different performance and some excelling in just few of them, but often at different operating conditions. Proper performance assessment can be done through figures of merit able to summarize the typical SPAD performance (i.e. photon detection efficiency, dark counting rate, afterpulsing probability, hold-off time, and timing jitter) and to identify a proper metric for SPAD comparisons, when used either as single pixel detectors or in imaging arrays. We present a comparison among some imager architectures and SPAD detectors and arrays in either photon-counting, timing, or imaging applications.