Categorical variables that are exclusive and exhaustive at the scale of modeling are commonly used to improve the geological model. Facies, alterations, lithologies and rock types are all used as categorical variables. Often, the major heterogeneity is in rock types or facies. The uncertainty in these categorical variables is best quantified through simulation which can incorporate uncertainty in category proportions and boundaries. Lessons in categorical simulation are aimed at the simulation and checking of categorical variables, but there is some overlap with continuous simulation.