This chapter discusses osteitis, where osteomyelitis is an imperfect pathophysiologic analogy for sinonasal osteitis. The term osteomyelitis refers to bony infection within the marrow space. Lamellar bone is marked by highly organized parallel layers, lamellae, of mineral crystals and collagen fibers that are slowly formed. The strong association of eosinophils with osteitic bone and recalcitrant disease offers a compelling explanation for sinonasal osteitis. Bone is a dynamic tissue that remodels in response to both biochemical and mechanical stimuli. Surgical intervention is a source of trauma that is known to predispose to osteitis. Mesenchymal stem cells localize to the site of injury and are stimulated to divide and differentiate toward chondrogenic or osteogenic lines. The recalcitrant inflammation associated with osteitic bone has yet to be associated with intraosseus bacteria. Multiple studies have demonstrated the association of osteitis with nasal polyps, asthma and eosinophilia.