Since the rediscovery of spiral-shaped gastric bacteria in 1983 by Warren and Marshal numerous detailed facts have enlarged our knowledge base for a better understanding of gastritis. The WHO classified Helicobacter as a class 1 carcinogen. Helicobacter plays a role in many diseases of the upper GI tract (gastric and duodenal ulcer, MALT lymphoma, and gastric cancer). Knowledge of the interaction between bacterium and host led to an improvement of the diagnosis of gastritis. Unfortunately endoscopy, histology, and patient's symptoms do not correlate with each other. Therefore, gastritis is a purely histological diagnosis. A histological diagnosis always needs to be accompanied by an etiological cause of the inflammation. Only when this information is given a clinical consequence can be drawn from a histological diagnosis. In Germany the so-called ABCD scheme of gastritis is widely used (A: autoimmune gastritis, B: bacterial gastritis, C: chemical reactive gastritis, D: other forms of gastritis). At least two antrum and two corpus biopsies (matrix diagnostics) are necessary to reach representative tissue with an acceptable probability. If less than these four (minimal standard) biopsies are taken, the histological diagnosis will remain uncertain.