Cracking in single-span concrete bridge decks was investigated. An assessment of the cracking as it relates to in situ concrete strength was performed. In cracked decks, concrete was of uniform quality and the compressive strength obtained from cores was in excess of the design strength. Evaluation of cores using an optical technique revealed that the cracks in the bridge decks formed at an early age, when the strength of the cementitious matrix is less than the strength of the coarse aggregate. Tests on mature concrete obtained from bridge decks indicate that the cementitious matrix is stronger than the coarse aggregate, resulting in aggregate fracture under tensile loading. Findings indicate that transverse cracking in the midspan portion of bridges with elastomeric bearings and longitudinal cracking at the supports in bridges with integral abutments result from restraint of early-age thermal movement of the bridge deck. The available evidence suggests that cracking in the bridge decks occurs in the first 48 h after casting. © 2016 American Society of Civil Engineers.