The behavior of concrete in biaxial compression is briefly described as determined from experiments performed on six mixes by a testing machine capable of loading concrete slabs simultaneously in two orthogonal directions. Strain readings in the three principal directions, microcracking, and ultimate load were observed and measured for various values of lateral load on specimens from each mix. The results show that the elastic parameters, the load at which microcracking occurs, and the ultimate load are not constant but are dependent on the value of the lateral load. The effect of varying the aggregate size, water-cement ratio, and aggregate-cement ratio was also studied; these were shown to have a considerable influence on the properties of concrete in biaxial compression.