We present an overview of the current status of input/output (I/O) on the Cray XT line of supercomputers and provide guidance to application developers and users for achieving efficient I/O. Many I/O benchmark results are presented, based on projected I/O requirements for some widely used scientific applications in the Department of Energy. Finally, we interpret and summarize these benchmark results to provide forward-looking guidance for I/O in large-scale application runs on a Cray XT3/XT4.