DIRT is a Java applet for modelling astrophysical processes in circumstellar dust shells around young and evolved stars. With DIRT, you can select and display over 500,000 pre-run model spectral energy distributions (SEDs), find the best-fit model to your data set, and account for beam size in model fitting. DIRT also allows you to manipulate data and models with an interactive viewer, display gas and dust density and temperature profiles, and display model intensity profiles at various wavelengths.