Cobalamin deficiency causes megaloblastic anaemia and/or neurological disorders. Mild to moderate deficiency conditions are associated with several age-related diseases. The availability of modern biomarkers and the cost-effective prevention via supplementation have dramatically reduced clinically manifested cobalamin deficiency. Several clinical conditions are likely to be caused by cobalamin deficiency or their progress can worsen if cobalamin deficiency co-exists. Testing cobalamin markers in patients with anaemia is necessary even when megaloblastic cells are not detected or mean corpuscular volume is normal. Studies testing cobalamin content in human milk have provided valuable information on determinants of milk cobalamin as a sole source of this nutrient in breastfed infants. The explanation and impact of abnormal cobalamin markers differ across age groups, populations, and background diseases. The chapter discusses four primary cobalamin biomarkers including total cobalamins, holotranscobalamin, methylmalonic acid, and homocysteine. Folate and cobalamin deficiencies can cause macrocytic anaemia. Patients with anaemia should be screened for nutritional deficiencies, among other possible causes.