Computational scientists need guidelines on best I/O practices, in order to make best use of existing I/O infrastructure. This book presents a detailed overview of Parallel I/O on current petascale platforms, including I/O hardware resources and filesystems at leading HPC facilities in the US. It covers the leading Parallel I/O libraries and includes case studies that illustrate the use of these libraries for conducting scientific computing. The book concludes with an overview of exascale trends and implications for parallel I/O.