Detailed information on the cracking of loaded concrete beams and slabs is now available as a result of recent investigations at the National Bureau of Standards. The tests results promise to prove useful in controlling the spacing and width of cracks in the future design of reinforced concrete structures. In general, the results show that width of cracks can best be controlled by using a large number of small reinforcing bars and by increasing the reinforcement. While cracks barely wide enough to be visible may be objectionably only because of appearance, cracks of greater width can be dangerous because of the possibility of corrosive agents attacking the steel reinforcing bars. Excessively wide cracks can also result in leakage in dams, tanks, and pools. The Bureau findings- that is, by increasing the bond strength between concrete and steel with improved reinforcing bars, widths of cracks may be reduced- will therefore make a significant improvement in reinforced concrete structures.