Internal cracks caused by high temperature or excessive moisture during maize (Zea mays L.) kernel development were characterized, and their effects on kernel quality were assessed. Pre-harvest stress cracks are often located near the middle of the kernel along the embryo axis, but they were also detected in other positions, irrespective of the shape of the kernel. X-ray analysis enabled visualisation of stress cracks that are invisible to the human eye and, therefore, gave a better estimate of the percentage of cracks. However, low temperature scanning electron microscopy of the surface of milled kernels revealed small cracks not noticed by visual or X-ray inspection. All kernels tested in this way had a crack of some sort in the endosperm tissue. Cracks were also frequent in the scutellum, but rare in the embryo axis. Endosperm cracks followed the boundary of the starch granules, but did not extend into the pericarp tissue. In contrast to external cracks caused by mechanical impact, pre-harvest internal stress cracks generally are not detrimental to germination and vigour. However, if the crack is located inside or perpendicular to the embryo axis, it may affect the quality of the kernel, probably by impeding nutrient translocation to the embryo.