A general overview is given of the causes of anemia with iron deficiency as well as the pathogenesis of anemia and the para-clinical diagnosis of anemia. Anemia with iron deficiency but without overt GI bleeding is associated with a risk of malignant disease of the gastrointestinal tract; upper gastrointestinal cancer is 1/7 as common as colon cancer. Benign gastrointestinal causes of anemia are iron malabsorption (atrophic gastritis, celiac disease, chronic inflammation, and bariatric surgery) and chronic blood loss due to gastrointestinal ulcerations. The following diagnostic strategy is recommended for unexplained anemia with iron deficiency: conduct serological celiac disease screening with transglutaminase antibody (IgA type) and IgA testing and perform bidirectional endoscopy (gastroscopy and colonoscopy). Bidirectional endoscopy is not required in premenopausal women