I. INTRODUCTION Most scanning electrochemical microscopy (SECM) experiments are conducted in the amperometric mode, yet microelectrodes have for many years been used as potentiometric devices. Not surprisingly, several SECM articles have described how the tip operated in the potentiometric mode. In this chapter we aim to present the background necessary to understand the differences between amperometric and potentiometric SECM applications. Since many aspects of SECM are covered elsewhere in this monograph, we have focused on the progress made in the field of potentiometric microelectrodes and presented it in the context of SECM experiments. Starting with an historical perspective, the key discoveries that facilitated the development and applications of micro potentiometric probes are highlighted. Fabrication techniques and recipes are reviewed. Basic theoretical principles are covered as well as properties and technical operational details. In the second half of the chapter, SECM potentiometric applications are discussed. There the differences between the conventional amperometric mode are developed and emphasized.