In water well drilling the clogging of the formation due to drilling fluid invasion is a process of which most engineers/ geologists are aware but may not fully understand. Some simple laboratory test rigs are described. These were used to examine the damage mechanisms and damage potential of several drilling fluids. This work did not include a study of damage removal. The results show that solids content and rheology are the main factors influencing the type and degree of damage, as defined by the location and amount of permeability reduction. Whilst skin damage is the most obvious and occurs with many fluids, internal damage is equally significant and could be the most difficult to remove. With internal damage it is important to remember that the fluid which introduces the damage is not usually the same as that used to try and remove it.