Petroleum fingerprinting is an invaluable tool in forensic geochemistry. This article summarizes applications of fingerprinting in several oil spills and natural oil seepages that we have studied during the last 25 years. It shows how each unique chemical fingerprint can be used to correlate or differentiate oils. Fingerprints can provide information about processes in the environment that impact oils such as weathering and microbial degradation. They can be used to evaluate organic matter that contributed to oils, and classify oils with regard to the geological framework of their source, such as evaluating geological facies, age, lithology, and depositional environment.