Liquid-fed direct methanol fuel cells (DMFCs) are one of the most promising candidates for portable power electronics and automotive applications due to their potentially high-energy density, simple storage, and distribution of the fuel. The concentration of methanol in the fuel circulation loop of a DMFC system is an important operating parameter, because it determines the electrical performance and efficiency of the system. The methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor for the purpose of maintaining optimal power and efficiency in the DMFC system. Various methods of sensing methanol concentration have been proposed over the past decade. This paper reviews these methanol concentration sensors for DMFCs, which are generally classified into two groups: electrochemical and physical. The construction and operating principles of each sensor, as well as its advantages and disadvantages, are described. The sensorless methods for controlling the methanol concentration are introduced briefly. Finally, the perspective on the future of methanol concentration sensors is discussed. Crown Copyright (c) 2005 Published by Elsevier B.V All rights reserved.