A systematic study of the counterion layering phenomenon at high surface charge in a planar electric double layer is reported. Extensive canonical Monte Carlo simulations are performed on a 1:1 electrolyte to investigate the effect on layering due to a variation of electrode charge density, electrolyte concentration, and ionic size. For a I mol dm(-3) salt at room temperature and ionic diameter 400 pin, the second layer begins to be formed at around a surface charge density of similar to0.4 cm(-2). It is seen that the variation of ionic size (at constant surface charge and salt concentration) leads to the most pronounced changes in the counterion layering. At an ionic diameter of 600 pm a third layer of counterions is observed. The simulations are extended to asymmetric valency 2:1 and 1:2 salts where layering effects are observed for monovalent counterions only. The calculated double layer capacitance is found to decrease in the presence of layering. A mean-field Poisson-Boltzmann equation coupled with a recently derived ionic exclusion volume term is found to predict this layering in qualitative agreement with the simulations. The use of a local concentration approximation improves the agreement in most cases.