A wide variety of insults including infectious agents, medications and endogenous and environmental chemical substances may cause injury to the gastric mucosa. This may take the form of inflammation (gastritis), reactive changes without inflammation (termed gastropathy by some authors), or a combination of the two. The histological appearances may be a ‘pattern’ of injury that is non-specific and can be seen in association with several aetiologies, or there may be histological features that are highly characteristic of a single injurious agent. In this chapter, ‘gastritis’ is considered in three sections – patterns of injury (with many potential causes), specific types of gastritis, and gastric mucosal injury related to medical therapies. A practical approach to diagnosis including potential diagnostic pitfalls is emphasised, as is the need for accurate endoscopic and clinical information when interpreting these specimens. Common entities, such as Helicobacter pylori gastritis and reactive gastritis, as well as rare conditions and other infections are considered. The discussion focuses on endoscopic biopsies (the most common specimen seen in practice) but these changes may be present in the resected stomach in the setting of both benign and malignant disease and also, increasingly, in specimens removed at the time of bariatric surgery.