Similarly, in untargeted metabolomics, each of the libraries provides complementary MS/MS data and pieces of information. For example, the commercial NIST small molecule spectral library predominantly contains human and plant metabolites, ReSpect contains plant metabolites, and the commercial METLIN library historically contained a significant proportion of lipids and dipeptides (full details on the current composition after its explosive growth are unknown as the library and information on the molecules that are part of the library have not been released publicly). The GNPS libraries historically focused on natural products, but they have since grown to include many major publicly available reference libraries, including lipids, drugs, pesticides, primary metabolites, food derived metabolites, common contaminants, and microbial metabolites. Furthermore, these libraries are exchanged with MoNA, MassBank EU, and other resources, such that they are not only leveraged in the GNPS analysis ecosystem but also by other analysis systems such as MZmine , MS-DIAL , and others. This broad sharing of spectral libraries ensures that untargeted metabolomics analyses can be performed against the largest possible spectral libraries, irrespective of the analysis platform. It should be noted that some spectral libraries, such as NIST and METLIN, are exclusively obtained in a single lab under more consistent experimental conditions, whereas other spectral libraries, such as MoNA and GNPS, are aggregated from community contributions and contain data that has been acquired in multiple labs, using different instruments, instrument platforms, and experimental protocols, and thus are more heterogeneous. Some metabolomics spectral library resources do not only include direct experimental MS/MS data from pure reference compounds, but also MS/MS spectra that were obtained using computational tools. For example, the MoNA and HMDB spectral libraries are augmented with in silico MS/MS spectra that were simulated using e.g. LipidBlast (MoNA) and CFM-ID (HMDB) (F. . Additionally, NIST provides smaller spectral libraries focused on specific types of molecules, such as oligosaccharides and acylcarnitines , that were annotated using analog searching -a strategy to identify structurally related molecules that differ by a modification by using a very wide precursor mass window (on the order of 100s Da)-rather than by measuring pure reference standards. GNPS contains secondary reference MS/MS spectra that have been annotated by high-quality matching against the NIST spectral library and "nearest neighbor suspect" MS/MS spectra that were obtained by propagating annotations using molecular networking across all public untargeted metabolomics data in the GNPS/MassIVE repository. By propagating annotations from existing spectral libraries to related MS/MS spectra it becomes possible to provide annotations that would otherwise not be accessible to the community. Therefore, these strategies expand the set of putative annotations that can be obtained in untargeted metabolomics experiments. This is especially relevant for molecules for which pure standards are not available, because their structures have never been synthesized or isolated from biological material, or because they cannot be described as a structure (e.g. sodium formate clusters or a specific modification of unknown regio-or stereochemistry). However, because the MS/MS spectra are not directly measured from pure reference material, additional care should be taken when interpreting annotations that match such library spectra. In other words, the user has to verify whether the annotations match the data and whether they make sense in the context of the experiment before investing precious time and resources to perform additional validation experiments.