The mechanical properties of aggregates may or may not have an effect on the strength or durability of concrete. Since concrete is an assemblage of individual pieces of aggregate bound together by a cementing medium, its properties are based primarily on the quality of the cement paste. Its strength is dependent also on the bond between the cement paste and the aggregate. If either the strength of the paste or the bond between the paste and the aggregate is low, a concrete of poor quality will be obtained irrespective of the quality of the aggregate. But when cement paste of good quality is provided, and its bond with the aggregate is satisfactory, then the mechanical properties of the aggregate will influence the strength of the concrete. The tests considered in this paper are applied ordinarily to coarse aggregate or to rock from which aggregate is produced. Some of these tests, particularly the toughness and hardness tests, may not be mentioned in present-day specifications for the quality of rock. However, the hardness test does appear suitable for determinations of the rate of wear of concrete prepared with various types of aggregate. Of special consideration may be concretes prepared with natural and lightweight aggregates. Studies recently made in the laboratories of the Bureau of Public Roads have shown that the Deval abrasion test is more suitable than the Los Angeles abrasion test for determining the quality of lightweight aggregates. And, of course, the Deval abrasion test is still used by some states to determine the quality of natural aggregates. Little immediate use can be seen for the toughness test, but it is described for the benefit of those who read of it in old publications or who may find requirements for toughness in present-day specifications. The present studies of increasing the resistance of concrete to impact by the addition of fine steel wires or nylon fibers to the concrete may cause an increase in interest of a toughness test for concrete, possibly based on the design of the test for rock. The method of test for toughness of rock certainly should not be dismissed as archaic.