R package IsoplotR: Statistical Toolbox for Radiometric Geochronology. An R implementation of Ken Ludwig’s popular Isoplot add-in to Microsoft Excel. Currently plots U-Pb data on Wetherill and Tera-Wasserburg concordia diagrams. Calculates concordia and discordia ages. Performs linear regression of measurements with correlated errors using the ’York’ approach. Generates Kernel Density Estimates (KDEs) and Cumulative Age Distributions (CADs). Future versions will include functionality for the Ar-Ar, Rb-Sr, Sm-Nd, Re-Os, U-Th-He, fission track and cosmogenic nuclide methods, including isochrons, age spectra, ternary diagrams, radial plots, banana diagrams and multidimensional scaling plots. A graphical user interface is provided as an RStudio Shiny app at http://isoplotr.london-geochron.com. Offline access to this interface will be provided at a later point in time.