Hydrogen jet flames, as a consequence of their greater reactivity, have a number of characteristics that are different from other flames. These are reviewed, as are the various attempts that have been made to characterise, and generalise, the lift-off distance at the base of jet flames. There has been a consistent improvement in the accuracy of the expressions for the prediction of lift-off distance. In addition to the greater reactivity that creates a small laminar flame thickness, allowance must be made for the significantly smaller air requirement for each mole of hydrogen fuel. The evolution of an expression for the lift-off distance is discussed and a new expression is provided. Alongside the blow-off data for lifted flame regimes, this enables hydrogen lifted flames to be fully characterised.