Course grained models (CGM) consist of replacing an atomistic description of a molecule with a lower-resolution model that averages away some of the fine details of the interactions. Numerous coarse-grained models (differing in the metric used to obtain the course-grained potential, such as the fitting to the forces or to structural features, but also in the definition of the course grain) have been developed in order to investigate the longer time- and length-scale dynamics that are critical to many long-chain molecular processes, such as polymer, lipid membranes, and proteins. Coarse graining can also refer to the removal of certain degrees of freedom (e.g., vibrational modes between two atoms) by freezing the bonds, bends, or torsional degrees of freedom, but more typically it implies that two or more atoms are collapsed into a single particle representation (the so-called united atom model was one of the first popular coarse grained models). Fundamentally the level to which a system may be coarse grained is bound by the accuracy in the dynamics and structural properties desired from a simulation. We refer to several review articles that provide explicit details.