This paper is taken from the 1953 J. J. Mattiello Lecture. An outline is given of the problems and techniques involved in the study of stress-strain relations of films. Reference must be made to original sources (13 references), if full utilization is to be made of the data in 27 figures. An excellent outline of the subject, not an extensive review of the literature. Perhaps most significant remarks refer to the influence of particle size, volume, and shape on film properties. Nodular and acicular shaped pigments cause different crackle pattern on aging. Nodular gives crow foot pattern; acicular, cracks in the direction of brush stroke. A reactive pigment, zinc oxide, produced films stronger than an inactive one, titanium oxide. -- AATA