Highly scaled CMOS devices are susceptible to atomic-level defects consisting of a single broken bond, misplaced atom, single-atom contaminant, etc. These types of defects impact a CMOS transistor worse as the transistor is scaled. This chapter discusses commonly known single-atom defects and their impact on CMOS chips. The silicon-silicon dioxide system is discussed, along with the defects and electrical effects of this system. The additional defects in a high-k metal oxide gate dielectric are discussed. Critical phenomena created by single-atom defects are discussed, including stress-induced leakage current (SILC) and giant random telegraph noise (RTS). These electrical defects can have a severe impact on CMOS devices, as the current magnitude of the defect can be of the same order of magnitude as the transistor currents, creating excessive noise and leakage.