This comment points out that the field electron emission (FE) equation given in the text of the cited review paper is a simplified version of the original 1928 theory developed by Fowler and Nordheim. This 1920s theory severely underestimated exchange and correlation effects (usually modelled as an image potential energy). A revised theory, published by Murphy and Good (MG) in 1956, showed that including image effects leads to predictions of current density that typically are higher than 1920s theory by a factor of 100 or more. Hence, to describe FE, it is currently better working practice to use MG theory (either in its original form or in a modern form), rather than 1920s type theory.