int? parameter

Hi,
I am inheriting some C#/Asp code from a programmer that believes in not comments at all. There are a few functions that have a parameter that is int? and I can't seem to find a reference to this anywhere as to what exactly this means. For example:

public AcitonResult Create(int? data_type_id, int? department_id, string[] CountyList){ ...}mentBasisList,

Can anyone tell me what exaclty placing the ? on the int type does?
 
Thanks,
Rod