  The success of high energy physics programs relies heavily on accurate
detector simulations and beam interaction modeling. The increasingly complex
detector geometries and beam dynamics require sophisticated techniques in order
to meet the demands of current and future experiments. Common software tools
used today are unable to fully utilize modern computational resources, while
data-recording rates are often orders of magnitude larger than what can be
produced via simulation. In this paper, we describe the state, current and
future needs of high energy physics detector and beamline simulations and
related challenges, and we propose a number of possible ways to address them.
