Blepharitis is probably the most common disease entity seen in the general ophthalmologist's office. A significant proportion of these cases are secondary to meibomian gland disease. This review outlines our knowledge of the histopathology, lipid abnormalities and role of microorganisms in meibomian gland dysfunction. We will also review the physiology of meibomian gland secretion and present models of meibomian gland dysfunction which have enhanced our knowledge of this condition. The importance of diagnosing associated conditions such as aqueous tear deficiency, contact lens intolerance, rosacea, and seborrheic dermatitis is emphasized. Although this condition causes significant morbidity in the population, there are effective treatments available and these will be discussed.