This paper presents the results of experimental studies of the micromechanical behavior of concrete under different loading conditions. Cylindrical specimens of normal- and high-strength concrete were tested under uniaxial and confined compression. Cracks and pores in the concrete specimens were impregnated with an alloy that has a low melting point. At the stress of interest, this alloy was solidified to preserve the stress-induced microcracks as they exist under load and images from the cross sections of the concrete specimens obtained using scanning electron microscopy (SEM). Stereological analysis that interprets three-dimensional structures by means of two-dimensional sections was used on the computerized images to determine the density, orientation, and branching of the compressive stress-induced microcracks and the effect of confinement on microcrack behavior. The density and branching of the microcracks decreased as the confining stress increased. The confining stress had a pronounced influence on microcracks in the interfacial transition zone (ITZ) between the cement paste and aggregate. The amount of interfacial cracking decreased significantly as the confining stress was increased. Under uniaxial compression there were significant differences in the crack patterns observed in normal- and high-strength concretes. Under confined conditions the two types of concrete had similar microcrack patterns.