Apache FOP (Formatting Objects Processor) is the world's first print formatter driven by XSL formatting objects (XSL-FO) and the world's first output independent formatter. It is a Java application that reads a formatting object (FO) tree and renders the resulting pages to a specified output. Output formats currently supported include PDF, PCL, PS, SVG, XML (area tree representation), Print, AWT, MIF and TXT. The primary output target is PDF.

FOP uses the standard XSL-FO file format as input, lays the content out into pages, then renders it to the requested output. One great advantage to using XSL-FO as input is that XSL-FO is itself an XML file, which means that it can be conveniently created from a variety of sources. The most common method is to convert semantic XML to XSL-FO, using an XSLT transformation.