Patterns of tectum ornamentation in pollen of Quercus (oak trees, Fagaceae) are investigated using high-resolution scanning electron microscopy. Tectumornamentation is highly consistent at the infrageneric level and can be used to polarize character states within Quercus by comparison with other genera in Fagaceae. In particular, pollen data strongly suggest the recognition of an infrageneric Ilex group and, for the first time, allow definition of the set of taxa that comprise this group. The infrageneric Ilex group displays a relatively simple tectum ornamentation exclusively found in this group of oaks, in Fagus, and in extinct lineages related to Trigonobalanus, Colombobalanus, andFormanodendron. Such a simple type of tectumornamentation is also knownto represent an early developmental stage in infrageneric groups of Quercus that have otherwise complex ornamentation (Quercus and Lobatae). Ornamentation of the tectum in the infrageneric Cyclobalanopsis group can be derived from the plesiomorphic Ilex type showing little accumulation of secondary sporopollenin. In the infrageneric Cerris, Protobalanus, Quercus, and Lobatae groups, accumulation of secondary sporopollenin masks the basic Ilex pattern. The five distinct types of tectum ornamentation are in perfect agreement with published molecular phylogenies of Quercus. Thus, pollen ornamentation is a most valuable tool to identify members of the major infrageneric groups in Quercus and provides the basis for a reevaluation of the fossil record of Quercus.