Preemption in simulation modeling is the interruption of ongoing operations when trigger events occur, and the transfer of the resources involved to new activities. The new preemption capabilities added to the Stroboscope simulation system are illustrated by their application to two earthmoving examples. The first involves moving soil using two wheelbarrows and two laborers, one who loads and one who hauls. Preemption occurs when the return of an empty wheelbarrow interrupts loading the other wheelbarrow to start a new haul activity immediately. When the loading and hauling cycles are balanced, preemption improves productivity when the time to haul is less variable than the time to load. In the second example, two cranes unload barges that bring construction fill material for undersea land reclamation. When only one barge is currently unloading, it uses both cranes. The arrival of a second barge preempts the unloading of the first barge so that each barge is unloaded by a single crane. When unloading a barge is complete, and there are no other barges waiting to unload, its crane is reallocated to the barge that may still be unloading so it can use both cranes. Modeling the required multiple preemptions and reallocations of cranes dynamically illustrates the new preemption capabilities added to Stroboscope and how they can be used to model preemption in general.