Multiple sclerosis (MS) is among the most common chronic inflammatory diseases of the central nervous system. Although not curable, the constantly increasing armamentarium of disease-modifying drugs now allows control of disease activity in many patients. The humanized monoclonal antibody alemtuzumab is a powerful drug licensed for the treatment of MS. Upon binding to the CD52 surface protein on CD4 + and CD8 + T cells, B cells, and monocytes, circulating CD52 + cells are eliminated via antibody- and complement-mediated lysis, and a less autoreactive adaptive immune system is reconstituted. The efficacy of alemtuzumab in terms of both clinical and magnetic resonance imaging outcomes has been demonstrated in several phase II/III trials including long-term extensions and follow-up studies. Treatment response to alemtuzumab is strongest as long as active inflammation is the predominant pathophysiological feature, and it is becoming less efficacious in neurodegeneration-dominated later stages of the disease. Thus, the optimal placement of alemtuzumab within treatment algorithms of MS is crucial. The impressive efficacy of alemtuzumab is counteracted by a less favorable safety profile. Besides usually manageable infusion-associated side effects, development of secondary autoimmunity in almost half of treated patients is the most disconcerting risk of alemtuzumab. The high frequency, the delayed occurrence, and the potentially severe course of secondary autoimmune diseases require awareness and a close long-term monitoring of patients treated with alemtuzumab. Biomarkers that would allow prediction of treatment response to alemtuzumab on the one hand and identification of patients at risk for the development of secondary autoimmune diseases on the other are not yet available. Thus, the overall success of alemtuzumab treatment critically depends on the patient selection. The aim of this article is therefore, to characterize the significance of alemtuzumab in the treatment of MS with a focus on the selection of the optimal patient.