The values of a set of variables can be sampled and stored during a model run by using the history command. These variables can then be plotted versus step number or versus other histories. Histories can also be written to a file (with the history export command). Only one variable may be given per history command. History variables may be added at any time. The contents of all histories can be erased with the history purge command, and all histories can be deleted with the history delete command. A summary of all histories is printed by the history list all command.

All histories are sampled at a single sampling interval. By default, the sampling interval for the history mechanism is every 10 steps. The sampling interval can be changed with the history interval command. Different sampling intervals cannot be assigned for different history variables.