Long bone histology has often been used to study growth, life history, and physiology of several dinosaurs but only little is known about life and growth parameters of Stegosaurus. This work provides the first extensive study of the long bone histology of Stegosaurus. We sampled several long bones and scapulae of four almost complete skeletons of Stegosaurus from the Late Jurassic Morrison Formation in Wyoming/USA, which are housed in the Sauriermuseum Aathal near Zurich in Switzerland. Long bone histology of Stegosaurus contains longitudinal primary osteons in a matrix that consists of a combination of both woven bone and parallel-fibered bone in varying proportions. In the inner cortex, fibrolamellar bone dominates, indicating fast growth. Toward the outer cortex, parallel-fibered bone dominates the tissue, indicating a gradual slow-down in growth. Three histologic ontogenetic stages were established. In the subadult stage, growth is still fast, whereas it decreases in adult stage I, and ceases in adult stage II. Comparing the ontogenetic stages to the body size of the specimens, two groups were recognized. This grouping may reflect intraspecific variability, sexual dimorphism, or taxonomic differences. Among dinosaurs, mainly showing true fibrolamellar bone, the long bone histology of Stegosaurus is most similar to that of the small basal thyreophoran Scutellosaurus. Accelerated growth led to the larger body size of the derived thyreophoran Stegosaurus compared to the small Scutellosaurus. Differences to the bone histology of other dinosaurs suggest that Stegosaurus grew more slowly and possibly had a lower metabolic rate than other dinosaurs of its size.