Carbon nanotubes have unique structures, differing from other forms of carbon. They have therefore been studied in the last decade as an alternative catalyst support. A general consensus is that carbon nanotubes can promote electrocatalytic activities of supported metal catalysts. In addition, the fibrous structure of nanotubes may also provide a new electrode structure and improve catalyst utilization. In this talk, we will discuss the pros and cons of using carbon nanotubes in PEM electrodes, especially carbon nanotube arrays. Electrode structure optimization will be discussed also.