The red eye is a clinical problem that is encountered regularly in most primary healthcare settings. A wide spectrum of diseases may cause a red eye. Fortunately, most are relatively benign, but many potentially sight-threatening conditions may manifest in a similar way. From the history and examination, the primary care physician must be able to differentiate between features that make primary care treatment possible and high-risk features that necessitate immediate referral. This article includes a discussion on features that distinguish benign from sight-threatening causes of red eye. Unilateral red eye, pain (a deep ache), deep redness, decreased visual acuity and photophobia signify more sinister causes. The red eye has an extensive differential diagnosis. Some of the common causes are conjunctivitis, subconjunctival haemorrhage, episcleritis, scleritis, anterior uveitis and acute glaucoma. Generally, patients who present with red eye can be divided into two groups: those who can be treated at primary care level and those who need secondary or tertiary level care. Other distinguishing features include a pattern to the redness, the type of discharge, the presence of increased lacrimation and photophobia, as well as corneal haze. However, these are not always easily employed as differentiating factors. Therefore, this article lists specific and basic features which can be used to identify the various causes of the red eye.