When an oil or gas well is being drilled, some of the borehole fluid (mud filtrate) leaks into the formation, displacing the native reservoir fluid. This creates an invaded zone around the wellbore. The invading fluid will generally have a mobility and compressibility that differ from the formation fluid. The presence of the invaded zone will affect the pressure transients measured by a formation tester. In this paper, a model that includes an invaded zone is presented together with an analysis of its effect at each of the probes of a multiple probe formation tester. The results show that the properties of the invaded zone dominate the pressure transient measured at the sink probe, strongly influence the transient at the horizontal probe, and only modestly affect the vertical probe transient. A suggested modification to the current interpretation procedure (which does not consider the invaded zone) for determining the horizontal and vertical mobility is also presented. The application of this modified procedure is discussed using a model example.