Because of the apparent importance of the ELF in diagnosing "electrideness", we thought it apt to provide a review of this important tool used for topological analysis of the electron density. In a nut shell, the ELF can be calculated from terms that are related to the probability of finding two electrons with the same spin close to one another. The mathematical formula used to define the ELF is constructed in such a way that if this probability is low, the ELF will be large, approaching unity.