Hemoglobin is the protein in red blood cells that carries oxygen throughout the body. It is made up of globin chains and heme. Anemia occurs when hemoglobin levels drop below normal, reducing oxygen delivery to tissues. Anemia can be classified based on red blood cell size and color. Common causes include iron deficiency from blood loss or poor diet, vitamin deficiencies, chronic diseases that increase hepcidin levels, and genetic disorders. Investigation involves blood tests of iron levels, mean cell volume, and red blood cell morphology. Treatment focuses on replacing lost iron and addressing the underlying cause. Prevention involves diet, food fortification, and supplementation of at-risk groups.