This chapter provides an introduction to the techniques for modelling fluids in numerical simulations in general relativity. It provides a broadbrush picture of the commonly used descriptions of equations of state in GRMHD simulations. It also provides a summary of the state-of-the-art microphysical descriptions of compact objects, compatible with recent constraints from multi-disciplinary physics. This would be useful to students and researchers outside the nuclear astrophysics community who would like to have some knowledge of realistic equations of state that are currently being implemented in tabulated form into such simulations.