Introduction: Sources of Variation Variation is the deviation from intended or designed values for a structure or circuit parameter of concern. The electrical performance of microprocessors or other integrated circuits are impacted bytwo sources of variation. First, environmental factors are those which arise during the operation of a circuit, and include variations in power supply, switching activity, and temperature of the chip or across the chip. These variations depend primarily on architectural and operating decisions such as power grid design and component placement. Time-varying (temporal) variation in these environment parameters can be a significant design concern. Circuit robustness to noise, cross-talk, and time- and switching-related aging or reliability factors must be considered carefully during circuit design. In this chapter, we will focus on a second category of variation sources. Physical factors during manufacture result in struct