In Chapter 8, the statistical nature of electromigration is described. Along with understanding of the basic physical degradation mechanisms, this is an important area of research due to the need for extrapolations from simple test structures to the product level. One has to keep in mind that electromigration testing is usually done on single-link structures. In some instances, a few links are stitched together in a series or parallel fashion, but massive-scale studies with large interconnect arrays have not been implemented yet as a standard testing methodology. Only the application of very large test structures with an extended amount of interconnect links encompassing metal lines and contacts/vias can lead to the detection of “early” or “extrinsic” failures, which are the limiting factor in the extrapolation to product-level interconnect systems. The detection of these early failures in electromigration and the complicated statistical nature of this important reliability phenomenon have been difficult issues to treat for decades in the past. In Chapter 8, an innovative technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone Bridge are discussed, and both Al- and Cu-based interconnect technologies are described.