Isoplot is a flexible tool for the mathematical and graphical interpretation of radiogenic-isotope data, especially as applied to geochronology. Isoplot 3.7 and Isoplot 4.1 are “Add-In” programs that run only under Windows Excel versions 2003 and earlier (Isoplot 3.7), or only under Excel versions 2007 and 2010 (Isoplot 4.1). No versions of Isoplot can be run as a stand-alone program.