The principal effect of Helicobacter pylori infection is lifelong chronic gastritis, affecting up to 20% of younger adults but 50% to 80% of adults born in resource-rich countries before 1950.We conducted a systematic review and aimed to answer the following clinical questions: What are the effects of H pylori eradication treatment in people with a confirmed duodenal ulcer, a confirmed gastric ulcer, confirmed gastro-oesophageal reflux disease (GORD), confirmed non-ulcer dyspepsia, uninvestigated dyspepsia, localised B cell lymphoma of the stomach, and non-steroidal anti-inflammatory drug (NSAID)-related peptic ulcers? What are the effects of H pylori eradication treatment for preventing NSAID-related peptic ulcers in people with or without previous ulcers or dyspepsia? What are the effects of H pylori eradication treatment on the risk of developing gastric cancer? Do H pylori eradication treatments differ in their effects? We searched: Medline, Embase, The Cochrane Library, and other important databases up to September 2007 (Clinical Evidence reviews are updated periodically; please check our website for the most up-to-date version of this review). We included harms alerts from relevant organisations such as the US Food and Drug Administration (FDA) and the UK Medicines and Healthcare products Regulatory Agency (MHRA).We found 58 systematic reviews, RCTs, or observational studies that met our inclusion criteria. We performed a GRADE evaluation of the quality of evidence for interventions.In this systematic review we present information relating to the effectiveness and safety of the following interventions: effects of H pylori eradication in different populations; relative effects of triple regimens, quadruple regimens, and sequential regimens.