The article introduces IMD, a computer program designed for modeling the optical properties of multilayer films. IMD can handle films with any number of layers and thicknesses, and it includes a full graphical user interface. The program allows for simultaneous modeling with up to eight independent variables and parameter estimation using nonlinear least-squares curve fitting to experimental data. The computation methods and user interface are described, and examples are provided to illustrate IMD's unique capabilities in modeling, fitting, and visualization. IMD is available for free via the Internet and can be run on various platforms. The article also covers the physics and algorithms used in IMD, including the treatment of interface imperfections, graded interfaces, and instrumental resolution. Additionally, it discusses the user interface, which allows users to specify parameters, variables, and visualize results using the IMDXPLOR tool. Examples are given to demonstrate how IMD can be used to visualize multidimensional optical functions, optimize design parameters, analyze X-ray reflectance data, and determine optical constants with confidence intervals.The article introduces IMD, a computer program designed for modeling the optical properties of multilayer films. IMD can handle films with any number of layers and thicknesses, and it includes a full graphical user interface. The program allows for simultaneous modeling with up to eight independent variables and parameter estimation using nonlinear least-squares curve fitting to experimental data. The computation methods and user interface are described, and examples are provided to illustrate IMD's unique capabilities in modeling, fitting, and visualization. IMD is available for free via the Internet and can be run on various platforms. The article also covers the physics and algorithms used in IMD, including the treatment of interface imperfections, graded interfaces, and instrumental resolution. Additionally, it discusses the user interface, which allows users to specify parameters, variables, and visualize results using the IMDXPLOR tool. Examples are given to demonstrate how IMD can be used to visualize multidimensional optical functions, optimize design parameters, analyze X-ray reflectance data, and determine optical constants with confidence intervals.