Eculizumab is the first drug approved for the treatment of complement-mediated diseases, and current dosage schedules result in large interindividual drug concentrations. This review provides insight into the pharmacokinetic and pharmacodynamic properties of eculizumab, both for reported on-label (paroxysmal nocturnal hemoglobinuria, atypical hemolytic uremic syndrome, generalized myasthenia gravis) and off-label (hematopoietic stem cell transplantation-associated thrombotic microangiopathy) indications. Furthermore, we discuss the potential of therapeutic drug monitoring to individualize treatment and reduce costs.