Current status and prospect of miniature and small fuel cells for portable devices have been reviewed from the following viewpoints. (i) Key technologies for downsizing: two technologies of solid polymer electrolyte and direct methanol fueling promoted downsizing of fuel cells. Principles of the two technologies were also explained. (ii) Cell efficiency and methanol crossover: power generation efficiency was discussed based on a theoretical equation and in terms of overpotential. Especially, methanol crossover through the electrolyte membrane affects the overpotential. The mechanism was interpreted according to electrode reactions occurred at cathode. (iii) Anode reaction and electrocatalysts: methanol oxidation reaction was described with a major focus on a bifunctional mechanism. In addition, anode materials to accelerate the methanol oxidation were introduced. (iv) Cathode reaction and electrocatalysts: electroactive materials for oxygen reduction were presented. Moreover, cathode materials which exhibit tolerance for the methanol crossover were exhibited. (v) Development of electrolyte materials: surface modification and new com-partment construction of membranes were introduced in order to protect the methanol crossover. (vi) Fuel cell system: an extreme system of fueling and cathode exhaust recycling technology to enhance the cell efficiency and diminish the total size was demonstrated.