Purpose : Ocular adnexal and orbital infections are broadly divided into preseptal(periorbital) and postseptal(orbital) cellulitis by orbital septum. In this study, we investigated the difference between periorbital and orbital cellulitis regarding their pathogenesis, clinical manifestations, treatments, and prognosis. Methods : We reviewed medical records of 50 cases who were hospitalized in the Severance hospital due to orbital cellulitis from May 1995 to April 2004. Results : There were 32 males and 18 females. The mean age was $3.2{pm}3.5$ year. According to the result of orbital computerized tomography, 36 cases were periorbital cellulitis, 10 cases orbital cellulitis and 4 cases not diagnosed. The clinical manifestations of periorbital cellulitis are periorbital swelling(100%), fever(19%), orbital pain(6%), and chemosis(22%). On the other hand, those of orbital cellulitis are periorbital swelling(100%), fever(80%), orbital pain(60%), proptosis(20%), chemosis(70%) and limitation of eye movement(20%). The etiologies of periorbital cellulitis are sinusitis(14%), upper respiratory infection(8%), conjunctivitis (19%), skin wound(14%) and unknown(44%). The etiologies of orbital cellulitis are sinusitis (50%), upper respiratory infection(20%), and unknown(30%). The first line antibiotics used in the majority of cases were combinations of cefoxitin+aminoglycoside. 5 patients with orbital cellulitis taking cefoxitin+aminoglycoside had to change the medication into vancomycin or clindamycin. 3 patients with orbital cellulitis underwent operation while 1 patient developed bacterial meningitis. Conclusion : According to invasion of orbit, ocular adnexal and orbital infections are quite different in their pathogenesis, treatment and prognosis. As atypical cases may confound the diagnosis, prompt orbital computerized tomography is required for an accurate diagnosis.