Over the past decade, MS/MS small molecule spectral libraries have steadily increased in size to include hundreds of thousands to millions of MS/MS spectra and hundreds of thousands of compounds (Figure ). Some of the largest experimental small molecule spectral libraries that are currently available include both commercial libraries, such as the National Institute of Standards and Technology (NIST) tandem mass spectral library () and the METLIN Gen2 spectral library , and open spectral libraries, which also serve as aggregation sites for third-party community spectral libraries, such as the Global Natural Products Social Molecular Networking (GNPS) community spectral libraries (M. and Massbank of North America (MoNA; ). Additionally, mass spectrometry instrument vendors also provide commercial spectral libraries, such as mzCloud (). Excitingly, publicly and freely accessible MS/MS spectral libraries recently saw explosive growth (Figure ). There also exist many other, often subject-specific spectral libraries, including Massbank and Massbank EU (), the Human Metabolome Database (HMDB) , the RIKEN tandem mass spectral database (ReSpect) , the monoterpene indole alkaloid database (MIADB) , the Critical Assessment of Small Molecule Identification (CASMI) contest libraries , European Molecular Biology Laboratory-Metabolomics Core Facility (EMBL-MCF) , the Pacific Northwest National Lab lipids library , the National Institutes of Health natural products library , the Lichen Database (LDB) , fungal dereplication , Chemicalsoft , WEIZMASS , MSforID , the reverse metabolomics libraries , and many others. Barring access restrictions, these spectral libraries are also often integrated into the previous spectral library aggregation resources, such as GNPS and MoNA. In this case, the SPLASH (SPectraL hASH) mechanism, which assigns unambiguous, database-independent hashed identifiers to MS/MS spectra, can be a useful tool for provenance of spectral data and detection of duplicate spectra that are shared across multiple data resources , similar to how InChIKeys are used as chemical identifiers.