Silicon photomultipliers have been available for decades as a commercial part. Since the 2000s, there has been a marked increase in the development of SiPMs in CMOS processes due to their low cost. Following from the precedence of using perimeter-gated SPADs (pg-SPADs) in standard CMOS processes, we show that these can also be used to improve SiPMs. This chapter covers SiPM architectures and their use of pg-SPADs as their constitutive Microcell microcell. This chapter is largely based on its related publications (Shawkat et al. (2018) IEEE Trans Circuits Syst I (TCAS-I) 65(11):3830–3841; Habib et al. (2017) A tunable single photon avalanche diode pixel with improved time resolution In: IEEE Sensors, Glasgow, pp 1–3; Habib et al. (2016) Improved signal to noise ratio across the spectral range for CMOS silicon photomultipliers. In: 2016 IEEE Sensors, Orlando, pp 1–3; Habib and McFarlane (2015) A perimeter gated single photon avalanche diode based silicon photomultiplier as optical detector. In: 2015 IEEE 58th International Midwest Symposium on Circuits and Systems (MWSCAS), Fort Collins, pp 1–4), Copyright, IEEE.