The avoidance of wheat- and gluten-containing products is a worldwide phenomenon. While celiac disease is a well-established entity, the evidence base for gluten as a trigger of symptoms in patients without celiac disease (so-called 'non-celiac gluten sensitivity' or NCGS) is limited. The problems lie in the complexity of wheat and the ability of its carbohydrate as well as protein components to trigger gastrointestinal symptoms, the potentially false assumption that response to a gluten-free diet equates to an effect of gluten withdrawal, and diagnostic criteria for coeliac disease. Recent randomized controlled re-challenge trials have suggested that gluten may worsen gastrointestinal symptoms, but failed to confirm patients with self-perceived NCGS have specific gluten sensitivity. Furthermore, mechanisms by which gluten triggers symptoms have yet to be identified. This review discusses the most recent scientific evidence and our current understanding of NCGS.