Spatially resolved mass spectra acquired point by point or in a microscope geometry, then reconstructed into ion-intensity maps, constitute imaging mass spectrometry (IMS). A focused ionization probe—most often a matrix-assisted laser desorption/ionization (MALDI) laser, but also secondary-ion or ambient laser-ablation electrospray sources—is rastered across a tissue section or other surface; at each coordinate the mass spectrum reports local molecular composition ^[1]. The resulting images are therefore chemical maps indexed by mass-to-charge ratio, not optical micrographs ^[2]. Conceptual boundaries exclude bulk mass spectrometry of homogenates, optical microscopy without mass analysis, and non-imaging MALDI of discrete spots that are not assembled into a spatial grid. Essential features are a spatially defined ionization event, registration of spectra to sample coordinates, and a mass analyzer whose resolving power and speed match the imaging experiment ^[3]. MALDI IMS in particular records intact proteins, peptides, lipids, and metabolites in situ as molecular snapshots of the specimen ^[4].