The orbits are conical in shape and contain the globes, cranial and autonomic nerves, extraocular muscles, vessels, lacrimal gland and tear ducts, and fat and connective tissue. Seven bones make up the orbit and the periosteum of these bones borders the orbit. An extension of the periosteum, the orbital septum, covers the anterior margin of the orbit and attaches over the front of the tarsal plates. The periosteum and orbital septum together form a relative barrier against the spread of infection into the orbit. An inflammatory process is described as either periorbital or preseptal if it is localized to the skin and subcutaneous areas of the eyelid (i.e., anterior to the septum). An inflammatory process within the orbit is described as either orbital or postseptal (i.e., posterior to the septum).

Despite the barrier formed by the periosteum and septum, infection still reaches the intraorbital space. The most common origin is from the paranasal sinuses, with extension occurring through numerous tiny foramina in the lamina papyracea that separate the orbits from the ethmoid sinuses. Additionally, an extensive venous system surrounds the orbit and consists of valveless veins that can serve as conduits for infection into the orbit from the paranasal sinuses as well as from the soft tissues of the face. In a similar manner, infection can be transmitted from the orbit into the intracranial compartment, which is often catastrophic. Less commonly, orbital and periorbital infections can occur secondary to trauma, foreign bodies, or bacteremia. Detecting extension into the subperiosteal or postseptal space is important in that surgical intervention may be required, whereas preseptal cellulitis typically responds to antibiotics.