Anemia is a prevalent complication in patients with cancer, both at diagnosis and during treatment, with notable negative effects on quality of life and overall prognosis. Iron deficiency is the most common cause of anemia in the patients and can affect almost half of patients with solid and hematologic malignancies. The pathogenesis is complex and multifactorial, including bleeding, malnutrition, medications, and inflammation resulted from cancer and cancer treatment. In fact, either absolute or functional iron deficiency can occur. Most iron deficient cancer patients present with functional iron deficiency, a condition with adequate iron storage but insufficient iron supply for erythroblasts and other iron dependent tissues. Functional iron deficiency is the result of the cancer related cytokine release, but in absolute iron deficiency iron reserves are depleted leading to similar but often more severe symptoms of inadequate iron store. Current therapeutic options in cancer anemia consist of iron administration, erythropoietic stimulating agents, and blood transfusion. The latter should be administered to a minimum, because of problems regarding risks. Here, we present a review on the epidemiology, pathophysiology, clinical outcomes, and therapeutic options of iron deficiency as well as the effect of iron therapy on tumor progression in anemic cancer patients.