A radically new CCD development by Marconi Applied Technology has enabled substantial internal gain within the CCD before the signal reaches the output amplifier. With reasonably high gain, sub-electron readout noise levels are achieved even at MHz pixel rates. This paper reports a detailed assessment of these devices, including novel methods of measuring their properties when operated at peak mean signal levels well below one electron per pixel. The devices are shown to be photon shot noise limited at essentially all light levels below saturation. Even at the lowest signal levels the charge transfer efficiency is good. The conclusion is that these new deices have radically changed the balance in the perpetual trade-off between read out noise and the speed of readout. They will force a re- evaluation of camera technologies and imaging strategies to enable the maximum benefit to be gained form these high- speed, essentially noiseless readout devices. This new LLLCCD technology, in conjunction with thinning should provide detectors which will be very close indeed to being theoretically perfect.