  Past and current X-ray mission allow us to observe only a fraction of the
volume occupied by the ICM. After reviewing the state of the art of cluster
outskirts observations we discuss some important constraints that should be met
when designing an experiment to measure X-ray emission out to the virial
radius. From what we can surmise, WFXT is already designed to meet most of the
requirements and should have no major difficulty in accommodating the remaining
few.
