Data contained in an xml document are typically manipulated by a computer program (application) written in a suitable computer language. The output is displayed on a screen, printed on paper, or stored in a file to be recalled at an opportune time. Xml data processing and manipulation are best illustrated with reference to the extensible stylesheet language (xsl) introduced in Section 1.5.2 and discussed in detail in this chapter. Scientific computing programmers should make a correspondence between an xsl processor and a computer language interpreter or compiler.