Samples of both fossil and modern resins have been investigated by x-ray diffraction techniques. Most fossil resins (ambers) yield only diffuse x-ray diffraction patterns, though some (settlingite, Highgate copalite, and guayaquillite) have relatively sharp patterns. Many modern resins, including those from species of Pinus and other conifers such as Picea and Abies, and many of those from Protium and Bursera, and Styrax benzoin, give sharp and distinctive patterns. Some crystalline constituents of the resins which yield the x-ray diffraction patterns have been identified. The similarity between the Protium and Bursera patterns and those of Highgate copalite and guayaquillite may indicate a possible genetic relationship between these modern and fossil resins.