Mineral aggregates play an important part in the improvement of all types of roads. This is true for the low-cost as well as for the more expensive construction. For even though the broadest possible definition of mineral aggregates (which would include all mineral particles such as rock, sand, and clay occurring within the roadway) is not accepted, the art and science of roadbuilding has not progressed to the point where it is possible to build a satisfactory, all-weather road without utilizing selected or processed aggregates for some stage of the improvement. “Low-cost road” is a relative term. A road which might be considered “low-cost” in one location or under one set of circumstances would be considered “high-cost” in another locality or under other circumstances. For the purpose of this paper, a low-cost road will be considered as the minimum road which will adequately serve the local or feeder traffic of a given locality at all seasons of the year without requiring excessive maintenance, and whose expected life is equal to that of higher type roads designed to accommodate heavier traffic. Generally speaking then, this paper is confined to that class of highway in which a layer of predominantly granular material is placed on the natural soil to serve as a wearing course or a base course for a relatively thin flexible surface. The more common types of mineral aggregates used in road construction of this sort are described. The general requirements for materials for this type of road are presented and some practical suggestions for the economical utilization of locally available aggregates are made. Water-bound macadams, which are road surfaces constructed of broken stone or slag cemented into a solid mass by means of stone or slag dust and water, are also included in the discussion although this type of road is one of the oldest known and the requirements are well established.