RadDecay is an interactive program for displaying radioactive decay information for any of the radionuclides in the libraries. RadDecay comes with the standard Nuclide Library of 497 nuclides and two optional libraries (ICRP-38 and MIRD). Data displayed includes the halflives, radioactive daughter nuclides, probabilities per decay, and decay product energies for alphas, betas, positrons, electrons, X-rays, and gamma rays. With the optional libraries, RadDecay also generates displays, plots, and printouts for composite beta spectrum data. The Nuclide Library information is primarily based on data supplied by RSICC at Oak Ridge National Laboratory. RadDecay conducts decay calculations for a single nuclide or a set of nuclides. For a single nuclide, given an initial activity and decay time by the user, RadDecay will calculate the remaining activity of the nuclide and the activity of its progeny. The decay chain is calculated and displayed for as many as 20 generations. Identical daughter nuclides generated along different branches of the decay chain are alternately combined and displayed. Decay chain and decay curves can be readily displayed and printed. Users can display information by clicking on any nuclide in the displayed decay chain. RadDecay also conducts decay calculations for a set of nuclides and generates a summary report for viewing, printing and saving. The user also has the option to set the minimum activity reported. RadDecay searches its library and reports the corresponding nuclides for photons, alphas, betas, electrons, or positrons within user-defined energy and half-life ranges.