Despite its decreasing incidence overall, gastric cancer is still a challenging disease. Therapy is based mainly upon surgical resection when the tumour remains localised in the stomach. Conventional chemotherapy may play a role in treating micrometastatic disease and is effective as palliative therapy for recurrent or advanced disease. However, the knowledge of molecular pathways implicated in gastric cancer pathogenesis is still in its infancy and the contribution of molecular biology to the development of new targeted therapies in gastric cancer is far behind other more common cancers such as breast, colon or lung. This review will focus first on the difference of two well defined types of gastric cancer: intestinal and diffuse. A discussion of the cell of origin of gastric cancer with some intriguing data implicating bone marrow derived cells will follow, and a comprehensive review of different genetic alterations detected in gastric cancer, underlining those that may have clinical, therapeutic or prognostic implications.