FOP is the world's first print formatter driven by XSL formatting objects. It is a Java 1.1 application that reads a formatting object tree and then turns it into a PDF document. The formatting object tree, can be in the form of an XML document (output by an XSLT engine like XT or Xalan) or can be passed in memory as a DOM Document or (in the case of XT) SAX events.

FOP is part of Apache's XML project. The homepage of FOP is http://xml.apache.org/fop.