In anaemia oxygen normally transported to various tissues is insufficient to meet physiological needs. Production of insufficient, or abnormal, red blood cells (RBCs), or increased destruction of RBCs may lead to anaemia as RBCs transport oxygen. Anaemia is the most widespread condition in the world with an estimated quarter of the global population affected, and was likely also common in past groups. Most cases are acquired principally due to micro-nutrient deficiencies, and arise from blood loss and/or insufficient/abnormally formed RBCs, but anaemia can also be congenital. In these cases, conditions normally cause increased destruction of RBCs. Guidance on diagnosis of anaemia in individuals of all ages is provided using the approaches set out in Chapter 2, with consideration of the basic biological mechanisms that underlie lesion formation discussed in Chapter 3. A series of bullet points are used to summarise the core concepts covered.