The environmental scanning electron microscope (ESEM) has the ability to image both dry and hydrated materials, without the need for a conductive coating, unlike conventional SEM. This presents a unique opportunity to explore the structure and dynamic mechanical characteristics of food systems, including those in a moist state. We have developed a technique in which quantitative stress-strain relationships can be obtained whilst allowing simultaneous imaging, by ESEM, of the mechanical response of a sample. The results of in situ compression tests on dry and hydrated wheat flour breadcrumbs are presented and discussed. It was found that a maximum in the critical fracture stress occurred at intermediate moisture content (similar to 16%). ESEM micrographs demonstrate the differences in mechanical behaviour at three different moisture contents (nominally dry, 16% and 30%). Our findings suggest that ＇voids＇ in cell walls, along with discontinuities between starch granules and gluten in the crumb, play an important role in fracture initiation. Further evidence shows that voids may be bridged by ＇struts＇ and ＇strings＇ of matrix material, which may be factors in the control of fracture propagation.