We have demonstrated the use of acoustic microscopy to characterize hydrated Portland cement microstructure. High frequency (1 GHz) studies identified reaction zones around bubbles, ettringite formation, and effects of aggregate composition. Lower frequency images (200-400 MHz) have been used to characterize effects of aggregate composition on cement/grain interface. The information obtained from acoustic microscopy complements optical and scanning electron microscopy. We demonstrate the importance of acoustic microscopy as an important nondestructive in situ technique for characterization of concrete. A major, unique advantage of the scanning acoustic microscope technique is its capability of penetrating into the material, which allows us to study cracks, defects and other zones of weakness that might be hidden below the surface.