This report, authored by Kenneth R. Ludwig and published by the United States Geological Survey, introduces Isoplot, a program designed for radiogenic-isotope geochemists to plot and regress X-Y data. The program supports various decay schemes and can calculate isochron ages directly from regression lines. It offers features such as plotting symbols, error symbols, and polygons, and can handle data from the keyboard or ASCII files. Isoplot supports IBM-PC compatible computers with CGA, EGA, and Hercules graphics modes, and requires at least 512 kilobytes of RAM. The report covers installation, usage, and advanced features, including Yorkfit regression models, polynomial and spline curve fitting, and creating U-Pb concordia plots. It also discusses the importance of accurate error correlations and the limitations of 3-point isochrons. The program can output graphics to the screen or high-quality hard copy using HPGL-compatible plotters.This report, authored by Kenneth R. Ludwig and published by the United States Geological Survey, introduces Isoplot, a program designed for radiogenic-isotope geochemists to plot and regress X-Y data. The program supports various decay schemes and can calculate isochron ages directly from regression lines. It offers features such as plotting symbols, error symbols, and polygons, and can handle data from the keyboard or ASCII files. Isoplot supports IBM-PC compatible computers with CGA, EGA, and Hercules graphics modes, and requires at least 512 kilobytes of RAM. The report covers installation, usage, and advanced features, including Yorkfit regression models, polynomial and spline curve fitting, and creating U-Pb concordia plots. It also discusses the importance of accurate error correlations and the limitations of 3-point isochrons. The program can output graphics to the screen or high-quality hard copy using HPGL-compatible plotters.