Abstract The filter cake is critical in preventing reservoir invasion by mud filtrate. This is of particular importance in open hole completions where damage cannot easily be bypassed. However little is known about the selectivity of the filtercake in controlling which components will invade the reservoir. An EC funded consortium between Statoil, Schlumberger Cambridge Research, and Institut Frangais du Petrole has focussed on the fundamental mechanisms of formation damage in order to design wellbore fluids for minimizing fluid losses and optimizing reservoir productivity. Polymer invasion of the reservoir has been shown to have a great impact on permeability reduction. Possible damage with a typical polymer based drilling fluid was evaluated in terms of static and dynamic filtration, determining cake permeability, filtrate composition, and oil permeability reduction after mud exposure. The effects of polymers on filtercake properties and filtrate composition were found to be related to their incorporation into the filtercake and more specifically to their interactions with solids and solutes present in the fluid. Filtration conditions were also found to have significant effects. Measurement of polymer adsorption and retention in cores as a function of polymer structure, and formation saturation were also made to characterise the effects of mud filtrate on producing formations. This was correlated to the observation of rock invasion and damage using computed tomography (CT) imaging. The improved understanding of formation damage caused by polymers is used to discuss criteria for effective and non-damaging fluid loss polymer architectures.