In Chapters 1 and 2 we discussed the motivation behind xml formatting and introduced the basic xml grammar. In Chapter 3 we explained how an xml document can be processed according to a companion xsl program to produce desired output that can be stored in a file or displayed in a web browser. In science and engineering applications, we are primarily interested in arithmetic data manipulation and to a lesser extent on formatted display. In this chapter, we discuss numerical computation in the xml/xsl framework.