Mie Resonance Shell Fitting (MRSFIT) is a specialized Fortran program engineered for the detailed characterization of heterogeneous spherical particles featuring a concentric spherical shell. It precisely calculates the core and shell radius, along with their respective refractive indices, by fitting observed Mie resonances to theoretical predictions based on Mie theory for core-shell geometries. This capability is crucial for understanding the complex internal structures of composite particles. Developed by Benjamin Vennes and Thomas C. Preston, MRSFIT is a Fortran-based application specifically tailored to address the challenges of analyzing multi-layered particles, where standard Mie fitting programs for homogeneous spheres (like MRFIT) are insufficient. While powerful, MRSFIT requires that mode assignments are known prior to the fitting process, which can make it less straightforward than MRFIT for simpler, homogeneous systems. This program is built upon the theoretical framework detailed in Vennes and Preston (2019), ensuring a robust and scientifically sound approach to core-shell particle analysis. MRSFIT is an indispensable tool in atmospheric aerosol science, particularly for investigating the morphology and phase state of atmospheric particles that undergo dynamic changes, such as phase separation or the formation of organic shells due to processes like secondary organic aerosol (SOA) uptake. It provides unprecedented insights into the internal architecture of these particles, which is vital for accurately predicting their behavior, reactivity, and overall environmental impact within the atmosphere. The software is typically applied to experimental data obtained from advanced analytical techniques, including optical trapping combined with Raman spectroscopy. This software complements MRFIT, as MRFIT can be used to provide the initial mode assignments for the homogeneous core or for the particle before a shell develops. MRSFIT's unique focus on core-shell structures makes it particularly valuable for studying time-dependent processes where particle morphology evolves. While other Mie theory calculators exist for core-shell particles, MRSFIT stands out for its specialized capability in fitting observed resonance spectra, providing a precise method for structural elucidation.