Evidence for sexual dimorphism was investigated in the horncores of two ceratopsid dinosaurs, Triceratops and Centrosaurus apertus. A review of studies of sexual dimorphism in the vertebrate fossil record revealed methods that were selected for use in ceratopsids. Mountain goats, bison, and pronghorn were selected as exemplar taxa for a proof of principle study that tested the selected methods, and informed and guided the investigation of sexual dimorphism in dinosaurs. Skulls of these exemplar taxa were measured in museum collections, and methods of analysing morphological variation were tested for their ability to demonstrate sexual dimorphism in their horns and horncores. Skulls and isolated horncores of Triceratops and Centrosaurus were measured in museum collections, and analysed with the methods applied to the exemplar taxa. Considerable morphological variation was found in both species, and was suggestive of dimorphism in Centrosaurus, but was deemed not to be attributable to sexual dimorphism.