The ExaScale IO (ESIO) library provides simple, high throughput input and output of structured data sets using parallel HDF5. ESIO is designed to support reading and writing turbulence simulation restart files but it may be useful in other contexts. The library is written in C99 and may be used by C89 or C++ applications. A Fortran API built atop the F2003 standard ISO_C_BINDING is also available.