For the most part, forensic chemistry is very similar to analytical chemistry. Among the typical job functions of an analytical chemist are: performing qualitative and quantitative analysis; sampling, setting error limits, validating and verifying results through calibration and standardization; and interpreting data in proper context.²  Forensic chemistry does all these, certainly, but it differs from other types of chemistry in several ways, one of which is central to the topic of this chapter: where its samples come from and what that means for interpretation of those samples, that is, could they have come from the same source. Both qualitative and quantitative methods are familiar to analytical and forensic chemists, who will employ them to address these questions. At a more fundamental level, however, is a method that is used almost constantly, and perhaps subconsciously, in all methods of examination and analysis: comparison. A thorough knowledge of the comparative process can improve any chemist's analysis. For purposes of this chapter, a deeper consideration of the comparative process can also help forensic chemists overcome or at least mitigate two primary challenges to accurate interpretation of their analyses.