The periosteum is the outer cortical layer of junctional hoof tissue, and the periosteal reaction is a stimulation of the periosteum that produces an increase in osteoblast activity in the inner layer of the periosteum. It usually indicates the presence of a lesion, commonly due to inflammation, tumor, trauma, subperiosteal hemorrhage, or, of course, during normal growth and development. Most of the changes in the periosteal reaction cannot be visualized on x-ray and are only visualized on x-ray when a sufficient amount of periosteal new bone is formed. In the early stages, the periosteal reaction appears as a thin, linear, dense shadow of varying length parallel to the bone cortex, which may be separated from the bone cortex by a very narrow translucent gap. As the disease progresses, the periosteal new bone will gradually thicken and become clearer on the x-ray. Continuous periosteal reactions can appear as single-layer periosteal reactions, multi-layer periosteal reactions, dense periosteal reactions, and parallel needle-like periosteal reactions. Interrupted periosteal reactions can appear as wedge-shaped solid periosteal reactions, interrupted laminar periosteal reactions, interrupted pin-like periosteal reactions, daylight radiolucent periosteal reactions, and Codman’s triangle. The different types of periosteal reactions on X-ray represent different types of pathology, and the benign malignancy of the disease can be identified based on the type of periosteal reaction.