The bond between the aggregate and the matrix has a significant effect on the strength, permeability, and durability of concrete. It is probable that the bond is affected by chemical reactions at the surface of the aggregate and by the surface texture or other characteristics of the aggregate. There is no suitable direct method available for measuring this bond between the aggregate and the matrix. The strengths obtained in testing the mortar-making properties of fine aggregates (ASTM Method C 87), the compressive strength of concrete cylinders (ASTM Method C 39), and the flexural strength of concrete (ASTM Method C 78), and visual observations made in connection with these tests provide some information about the bond. High strengths are an indication of a good bond. When it is observed that breaks occur through the aggregate, there is evidence of good bond. When the break occurs around the aggregates and they can be pulled out of their sockets, a poor bond is indicated. The entire problem of the chemical and physical factors that influence the bond between aggregate and matrix is one that deserves further study.