A quantitative analysis of the steric effect of aliphatic groups was carried out from first principles. An intuitive framework is proposed that allows the separation and straightforward interpretation of two contributors to the steric effect: steric strain and steric shielding (hindrance). When a sterically demanding group is introduced near a reactive center, deformation of its reactive zone will occur. By quantifying this deformation, a convincing correlation was established with Taft's steric parameters for groups of typical size, supporting the intuitive image of steric shielding; bulky groups slow down the reaction by limiting the accessibility of the reactive centre. On the other hand, the strong initial repulsion between the reactant and the substrate by means of the filled-filled orbital interaction results in the deformation of the substrate as well as a less stabilizing reaction zone, which are the manifestations of the steric strain. We thus conclude that both steric strain and steric hindrance can be derived from the Pauli repulsion evolving between the reactants in the course of the reaction. © the Owner Societies 2012.