This chapter gives an overview of the file structure of persistent data3.1. Most of the data handled by NEMO is in the form of a specially designed3.2XML-like binary format, although exceptions like ASCII files/tables will also be discussed. Ample examples illustrate creation, manipulation and data transfer. We close this chapter with a few examples of function descriptions, a dataformat that make use of the native object file format of your operating system (a.out(5) and dlopen(3)).