An ultramicroelectrode is an electrode whose characteristic dimensions are smaller than 25 μm. They have been used in electrochemistry for the last 30 years and can be used in studying extremely small sample volumes. Due to these advantages, ultramicroelectrodes have revolutionized electrochemical applications and methodologies. They have also transformed the time/space accessibility of experimentation. Nanomaterials are materials whose dimensions are at the nanometer scale (1–100 nm) or assembled at this range. When compared to conventional materials the structure and properties of nanomaterials are essentially changed. They show chemical, physical, and electronic properties that are not seen in other materials or even materials from which the nanoparticles were prepared. Nanomaterials are used for electrode modification, also including ultramicroelectrodes at electrochemical sensing systems; among these nanomaterials, one can count carbon nanoparticles, noble metal nanoparticles, metal oxide nanoparticles, and bimetallic nanoparticles. This chapter examines the role of nanoparticles in designing ultramicroelectrodes and their applications. The concepts, geometries, fabrication, and advantages of ultramicroelectrodes are defined and discussed. Finally, the applications of nanoparticle-modified electrodes are presented in such a way as to display the advantages of modification as well as the ultra-small size of the electrodes.