Missing ET Performance in ATLAS

The observation and measurement of missing transverse energy in an event is a key signature in SUSY searches. This paper describes the reconstruction and calibration of missing transverse energy in ATLAS, and addresses the quality of the measurement in a variety of simulated processes which contain physical missing transverse energy such as $Z$ $\to$ $\tau^{+}\tau^{-}$, $A^{0}$ / $H^{0}$ $\to$ $\tau^{+}\tau^{-}$, $t\overline{t}$, and SUSY as well as processes which contain no missing transverse energy such as minimum bias and $Z$+jets. Expectations for the use of early data in evaluation of the measurement of missing transverse energy are presented.