A novel CCD has been commercially produced by Marconi Applied Technology, UK under the trade name of L3Vision, and by Texas Instruments, USA under the trade name Impactron, both of which incorporate an all solid-state electron multiplying structure based on the Impact Ionisation phenomenon in silicon. This technology combines the single photon detection sensitivity of ICCDs with the inherent advantages of CCDs. Here we compare the electron multiplying CCD (EMCCD) with scientific ICCDs. In particular we look at the effect of the Excess Noise Factors on the respective S/N performances. We compare QEs, spatial resolution, darksignal, EBI and Clock Induced Charge (CIC), with the latter two as the ultimate limitations on sensitivity. We conclude that the electron multiplying CCD is a credible alternative to ICCDs in all non-gated applications.