Design formulations for calculation of bond strength in structural concrete members usually include a number of parameters such as the stress acting in the reinforcement, the concrete cover and the diameter of the reinforcing bars. These parameters are actually necessary to account for geometric, strain and size effect that govern many bond issues and that have been experimentally verified. Unfortunately, most design approaches are still based on empirical formulas that do not lead to a clear understanding why the different phenomena have to be taken into account. In this paper, a mechanical explanation of such effects is presented with reference to the various potential bond failure modes. Analogies to other related problems (such as spalling of concrete cover in arch-shaped members) are finally discussed.