This article, which deals with the study of the microscopic modifications of DEF-affected materials, has two main objectives. The first one is to study the influence of sample preparation on the microcracks of specimens observed in Scanning Electron Microscopy (SEM). From the results of this study, it can be concluded that direct observations of the samples are inappropriate because they produce cracks that are not ascribable to the pathology. It is therefore preferable to use an indirect technique of observation such as the replica technique. This technique was developed in the 1980s and used to study the evolution of microcracking due to mechanical damage to the concrete. The results presented in this article show that it is possible to use this technique to study the microcracking associated with delayed ettringite formation. The second objective is to study ettringite formation during the swelling of DEF-affected specimens. The main results show that secondary ettringite is initially formed in the paste–aggregate interface even at low levels of expansion. For these levels of expansion, there is no trace of secondary ettringite or microcracks observable in the paste. However, for higher expansions, cracks filled with ettringite appear in the paste.