This chapter discusses acute sinusitis, which means acute infection or inflammation of the paranasal sinuses of less than four weeks duration. Sinuses develop as small diverticula from the nasal cavity which invade the surrounding bones of the skull. Pansinusitis is the term applied to inflammation of all the paranasal sinuses, whereas individual sinus involvement is named accordingly such as acute maxillary sinusitis, acute ethmoiditis, acute frontal sinusitis or acute sphenoiditis in order of occurrence. General diseases such as influenza, measles, whooping cough and pneumonia may lead to sinusitis. Sinusitis passes through five stages such as catarrhal stage, exudative stage, suppurative stage, stage of complications and stage of resolution. Inflammatory changes include hyperaemia with outpouring of serum and polymorphs associated with local swelling, redness and oedema. Specific infections are due to fungi, syphilis, tuberculosis and leprosy. Ethmoiditis may give rise to swelling at the inner canthus of same eye.