The subject of this H.R. Procter memorial lecture is animal skin structure, its chemical composition and the changes that occur in the beamhouse. Light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are used to determine the structure of the epidermis and dermis or corium, including detailed study of collagen fibre bundles (fibrils and subfibrils, covered with a proteoglycan, dermatan sulfate), the grain surface (grain enamel, hyaline layer), and the basement membrane (including lamina densa and lamina lucida). Sample preparation for tissue sectioning involving fixation with glutaraldehyde, dehydration and embedding in resin, results in some shrinkage; freeze-fracturing reduces shrinkage. Histochemical staining of cross-sections was done using ruthenium red and cupromeronic blue, or with specific collagen antibodies. Effects on the skin structure are described for industrial and other processes including exposure to trypsin or other proteolytic enzymes (from soaking or putrefaction), liming, bating and pickling, splitting up, and chrome tanning. Micrographs in the article illustrate structural features of bovine (cattle) hide, woolskin, firm and soft side leather, mature ox hide, calfskin, sheepskin, pigskin and nappa leather (from hair sheep).