This work reports on the physical definition and extraction of threshold voltage in Tunnel FETs (field effect transistors) based on numerical simulation data. It is shown that the Tunnel FET has the outstanding property of having two threshold voltages: one in terms of gate voltage, VTG, and one in terms of drain voltage, VTD. These threshold voltages can be physically defined based on the transition between a quasi-exponential dependence, and a linear dependence of the drain current on VGS or VDS, and by extension, on the saturation of the tunneling energy barrier width narrowing. The extractions of VTG and VTD are performed based on the transconductance change method in the double gate Tunnel FET with a high-k dielectric, and a systematic comparison with the constant current method is reported. The effect of gate length scaling on these Tunnel FETs' threshold voltages, as well as the dependence of VTG on applied drain voltage and VTD on applied gate voltage, are investigated.