The Smithsonian Astrophysical Observatory (SAO) in collaboration with SRI/Sarnoff has been developing monolithic CMOS imaging detectors that are intended for use as X-ray imaging spectrometers for over a decade. The ultimate goal of this joint effort is to produce X-ray Active Pixel Sensor (APS) detectors that are Fano-limited over the entire 0.1-10keV band while incorporating the benefits of CMOS technology. These benefits include: low power consumption, radiation “hardness”, high levels of integration, and very high read rates. A large format Fano-limited CMOS imager would be ideal for large “facility class” missions such as Lynx. Similarly, since CMOS devices are simple to operate (e.g require little to no cooling), they would also be ideal for smaller, resource limited, X-ray applications such as “SmallSat” missions. SmallSats represent more immediate opportunites to both demonstrate nascent technologies while carrying out meaningful but typically narrowly defined science missions. SAO and SRI have produced 1k by 1k format back-thinned, CMOS devices with 16μm pitch, 6 Transistor Pinned Photo Diode (6TPPD) pixels with ~135μV/electron sensitivity. The device known as the Big Minimal III (BMIII) is optimzed for X-ray photon counting applications. The high sensitivity pixel ensures that even low energy (100eV) x-rays produce a macroscopic voltage at the pixel. The detectors incorporate a versatile, parallel and therefore very fast signal chain. These BMIII detectors are fabricated on 10μm epitaxial silicon so the thickness to pixel pitch ratio is small; this increases the number of single pixel X-ray events.