Sub-threshold conduction is an important consideration when dealing with modern devices, especially due to the trend towards increasingly smaller device sizes. Shorter channels have adverse effects on sub-threshold swing, affecting device operation in this region. Analog designers would like a smooth and accurate model in order to properly utilize this highly efficient operating region, while digital designers would prefer to understand methods to minimize channel conduction when a device is in sub-threshold. This paper will review previously published works that discuss analytical models for different sub-threshold concerns, including short-channel effects and the effects due to barrier-lowering. Experimental data is also presented which verifies some of these selected models. Finally, areas for further research into this operating region will be presented.