Mie Resonance Fitting (MRFIT) is a sophisticated Fortran program designed for the precise characterization of homogeneous spherical particles. Its primary function is to calculate the radius and refractive index of a sphere by accurately fitting observed Mie resonances to theoretically predicted resonances using Mie theory. This makes it an indispensable tool for researchers working with single-particle characterization in various scientific disciplines. This Fortran-based software, developed by Thomas C. Preston and Jonathan P. Reid, offers robust capabilities for analyzing light scattering data. Version 1.1 of MRFIT includes enhanced features such as the calculation of linewidths and an option to perform a simple grid search, improving its versatility and ease of use. It provides critical mode assignment information, including mode number, mode order, and polarization. The program is capable of determining particle size with nanometer-level accuracy and refractive index with a precision on the order of 10^-4, making it highly suitable for demanding research applications. MRFIT finds extensive application in atmospheric aerosol science, optical physics, and material science. It is particularly valuable for interpreting experimental data derived from optical trapping setups coupled with spectroscopic techniques, such as Raman spectroscopy, where Whispering Gallery Modes (WGMs) are observed. The software's ability to precisely determine the physical properties of microspheres and droplets is crucial for understanding complex atmospheric processes, studying fundamental light-matter interactions, and advancing microfluidic technologies. A key benefit is its efficiency; once mode assignments are known, it rapidly determines best-fit parameters, bypassing time-consuming multi-dimensional searches. This digital tool is typically utilized in conjunction with experimental systems that generate Mie resonance spectra, such as advanced optical tweezers. While primarily designed for homogeneous particles, MRFIT can still provide accurate results for weakly absorbing materials, even though it is strictly valid for materials with negligible imaginary refractive index. It often serves as an initial analysis step for core-shell particles, providing essential mode assignments that can then be refined by more specialized software like MRSFIT.