Highway base courses are typically constructed using crushed and processed aggregate. The engineering characteristics of the various base course materials used on Montana highways have not been thoroughly investigated or quantified. Consequently, the designer must rely on experience and habitual practices, which often lead to inconsistencies in design and confusion about aggregate specifications. The permissible gradation range of these materials covers a range of particle sizes, leading to varied performance and characteristics, because particle size distribution, shape, and surface texture can impact the engineering properties. This research will study the engineering characteristics of three aggregate materials used on highway projects by the Montana Department of Transportation.