Anaemias are diseases, which are characterized by a decreased concentration of haemoglobin and erythrocytes in the blood. They can be classified on the basis of etiopathogenetic and laboratory-diagnostic principles. From this results a differentialdiagnostic model of thinking, which may relieve the differential diagnosis on the basis of anamnesis and careful clinical examination and with aimed use of laboratory primary and additional examinations. It is particularly to be taken into consideration that anaemias very often may be complication or symptom of most various basic diseases. To combat an anaemia "differential-diagnostically" ex juvantibus by means of iron and/or vitamin B12, must be generally refused.