The CHARMM force field is one of the most widely used atomistic force fields in biomolecular simulation. Currently, the force field covers proteins, nucleic acids, lipids, carbohydrates, ions, small molecules, and a wide range of modified (noncanonical) residues within each of these categories. Augmenting the core CHARMM force field is the CHARMM General Force Field (CGenFF), which can be applied to nonstandard species, most often ligands and drug-like molecules. In this webinar, the functional form and parametrization strategy of the CHARMM force field will be presented along with key features that can be exploited for simulations of complex systems. Other topics include the implementation of the force field in the GROMACS simulation software and details on how users can convert CGenFF-generated topologies into GROMACS format for use in simulations.