Early Cambrian olenelloid trilobites offer an ideal opportunity to study rates and patterns of evolution during a critical interval of trilobite (and metazoan) history. The ontogeny of olenelloids has been a neglected research area, despite the important phylogenetic insights that ontogenetic studies can yield. An ongoing investigation into olenelloid ontogeny highlights the wealth of information that can be obtained from paleobiological studies, and demonstrates the significance of ontogeny in phylogenetic reconstruction. Despite olenelloid disparity, four successive ontogenetic phases of cephalic development can be consistently recognized and were evolutionarily conserved (homologous) across the clade. Entry in to phases 2 and 4 were defined by marked alterations in growth patterns of the glabella, while entry into phase 3 was characterized by differentiation of the genal spines and consequent modifications to growth patterns of the extraocular area. Rates and patterns of ontogenetic shape change can be rigorously compared within this framework. A decrease in growth rate during phase 4 is evident in some taxa. Development of an opisthothorax in some taxa permitted generation of large numbers of thoracic segments (over 35 in at least 8 species). Intraspecific variation in opisthothoracic segment number (individuals of "Olenellus" fowleri differed in count by as many as 8 segments) render the holaspid period difficult to define. Release of opisthothoracic segments may have continued throughout life in these taxa. Enlargement of the pleurae and pleural spines of the third thoracic segment, where developed, was not associated with an elongated axial ring, and would not have interfered with transportation of food particles along the ventral median food groove. The work has also revealed several novel characters of potential phylogenetic importance and has clarified the nature of others (demonstrating previously unsuspected variance within and dependence among suits of characters), suggesting that current hypotheses of olenelloid phylogeny may be in need of revision. The study raises tantalizing questions regarding broader issues of trilobite paleobiology and evolution, particularly with respect to thoracic segmentation.