Endovascular aneurysm repair (EVAR) is nowadays a globally applied treatment of abdominal aortic aneurysms (AAAs). The Achilles heel of EVAR is the incomplete seal of the aneurysm sac (endoleak) or the persistence of significant pressure in the aneurysm sac without detectable endoleak (endotension). Therefore, follow-up is needed after EVAR. CT is considered the __gold-standard__ for the detection of endoleak and endotension. However, the CT has several drawbacks. Hence a new follow-up method is needed. This thesis contributes to the development of the rationale of aneurysm sac pressure (ASP) monitoring as follow-up. The aim of this thesis is to evaluate the possible pitfalls of ASP monitoring. The relationship between endoleak and ASP is not clear. Results of different studies are compared and the present knowledge about determinants of ASP is discussed. A model of the human circulation and thrombus analogues are developed and validated. The effect of the ASP measuring technique, the aneurysm sac thrombus, the sensor motion and the direction of ASP measurement on the measurement itself is evaluated. This thesis demonstrates that aneurysm sac pressure is not straightforward. A pressure trend seems more appropriate to follow than absolute aneurysm sac pressures.</p