Gastric adenocarcinomas are tumours of decreasing incidence in the Western world, although they are still the fourth leading cause of cancer mortality. The purpose of these clinical guidelines is to provide recommendations for the diagnosis and treatment of this disease based on the best available evidence. Regarding resectable gastric cancer, the various potential therapeutic options are discussed (adjuvant or perioperative chemotherapy, and adjuvant or neoadjuvant chemoradiotherapy). With regard to advanced or metastatic disease, different alternative combinations of conventional cytotoxic agents including a platinum agent (cisplatin or oxaliplatin) and a fluoropyrimidine (5-FU, capecitabine or S1), with or without a third drug (epirubicin or docetaxel), as well as their integration with new biological agents (trastuzumab in HER2+ tumours), are discussed. Finally, an outline is provided of the main lines of research and development of therapies for this disease.