AbstractSequencing complete tumor genomes and exomes has sparked the cancer field's interest in mutation signatures for identifying the tumor's carcinogen. This review and meta‐analysis discusses signatures and their proper use. We first distinguish between a mutagen's canonical mutations—deviations from a random distribution of base changes to create a pattern typical of that mutagen—and the subset of signature mutations, which are unique to that mutagen and permit inference backward from mutations to mutagen. To verify UV signature mutations, we assembled literature datasets on cells exposed to UVC, UVB, UVA, or solar simulator light (SSL) and tested canonical UV mutation features as criteria for clustering datasets. A confirmed UV signature was: ≥60% of mutations are C→T at a dipyrimidine site, with ≥5% CC→TT. Other canonical features such as a bias for mutations on the nontranscribed strand or at the 3′ pyrimidine had limited application. The most robust classifier combined these features with criteria for the rarity of non‐UV canonical mutations. In addition, several signatures proposed for specific UV wavelengths were limited to specific genes or species; UV's nonsignature mutations may cause melanoma BRAF mutations; and the mutagen for sunlight‐related skin neoplasms may vary between continents.