This book documents the history of John S. Dye’s counterfeit detectors, a task made difficult by the ephemeral nature of such publications. Because new counterfeits and new genuine bank notes were constantly being circulated, counterfeit detectors quickly became outdated and were discarded, leaving very few surviving copies. This book fills in the gaps, largely through advertisements and articles in digital databases of contemporary newspapers. Dye first published a counterfeit detector in Cincinnati in 1850. Over the rest of his life, at various times and in various places, he published at least four other counterfeit detectors, a bank note reporter, and a book on notorious counterfeiters. After Dye’s death in 1881, others completed a coin encyclopedia be had nearly completed and continued to publish a counterfeit detector bearing his name until 1910.