I thank R. Cifelli, R. Lupia, N. Czaplewski, K. Padian, A. Lee, and M. Brown for helpful comments and discussion. I thank H. Woodward and an anonymous reviewer, whose comments and questions improved the manuscript. I thank R. Cifelli (OMNH), P. Makovicky and W. Simpson (FMNH), J. Horner and E-T. Lamm (MOR), M. Norell and C. Mehling (AMNH), D. Winkler (SMU), T. Rowe, W. Langston, and L. Murray (TMM), and J. Gauthier and W. Joyce (YPM) for access to specimens. I thank the OMNH, University of Oklahoma College of Earth and Energy, and UCMP for use of thin-sectioning equipment. I am grateful to E-T. Lamm, R. Burkhalter, K. Davies, J. Larsen, R. Lupia, and J. Person for their invaluable assistance in preparing and photographing specimens, and to W. Langston and M. Wedel for providing comparative photographs of Tenontosaurus skulls. I thank M. O'Leary and R. Monk, who provided technical assistance and increased file size capabilities for my MorphoBank uploads. I am especially grateful to R. Cifelli, J. Horner, and P. Makovicky for allowing me to post high-resolution images of their specimens on MorphoBank, extended access to thin sections, and for their patience with me in seeing this project to publication. This work was completed in partial fulfillment of the requirements for the Master of Science degree, Department of Zoology, University of Oklahoma. ; Conceived and designed the experiments: SW. Performed the experiments: SW. Analyzed the data: SW. Contributed reagents/materials/analysis tools: SW. Wrote the paper: SW. ; Tenontosaurus tilletti is an ornithopod dinosaur known from the Early Cretaceous (Aptian-Albian) Cloverly and Antlers formations of the Western United States. It is represented by a large number of specimens spanning a number of ontogenetic stages, and these specimens have been collected across a wide geographic range (from central Montana to southern Oklahoma). Here I describe the long bone histology of T. tilletti and discuss histological variation at the individual, ontogenetic and geographic levels. The ontogenetic pattern of bone histology in T. tilletti is similar to that of other dinosaurs, reflecting extremely rapid growth early in life, and sustained rapid growth through sub-adult ontogeny. But unlike other iguanodontians, this dinosaur shows an extended multi-year period of slow growth as skeletal maturity approached. Evidence of termination of growth (e.g., an external fundamental system) is observed in only the largest individuals, although other histological signals in only slightly smaller specimens suggest a substantial slowing of growth later in life. Histological differences in the amount of remodeling and the number of lines of arrested growth varied among elements within individuals, but bone histology was conservative across sampled individuals of the species, despite known paleoenvironmental differences between the Antlers and Cloverly formations. The bone histology of T. tilletti indicates a much slower growth trajectory than observed for other iguanodontians (e.g., hadrosaurids), suggesting that those taxa reached much larger sizes than Tenontosaurus in a shorter time.