This article provides a perspective on collisions of ions with surfaces, including surfaceinduced dissociation (SID) and reactive ion scattering spectrometry (RISS). The content is organized into sections on surface-induced dissociation of small ions, surface characterization of organic thin films by collision of well-characterized ions into surfaces, the use of SID to probe peptide fragmentation, and the dissociation of large non-covalent complexes by SID. Examples are given from the literature with a focus on experiments from the authors' laboratory. The article is not a comprehensive review but is designed to provide the reader with an overview of the types of results possible by collisions of ions into surfaces. (J Am Soc Mass Spectrom 2008, 19, 190 -208) © 2008 American Society for Mass Spectrometry T andem mass spectrometry (MS/MS) is an essential tool for elucidating ion structure. The MS/MS experiment involves mass selection of a precursor ion followed by ion activation and subsequent dissociation. The ion activation step is commonly accomplished via collision-induced dissociation (CID) in which the initial kinetic energy of a projectile ion is converted into internal energy through inelastic collisions with a neutral gas. Several alternative activation methods have been used in tandem mass spectrometry, one of which is surfaceinduced dissociation (SID). SID is analogous to CID, except that a surface replaces the neutral gas as the collision target. A typical ion-surface collision event is illustrated in Figure 1.The incorporation of a surface into a mass spectrometer for ion activation was pioneered in the laboratory of R. Graham Cooks in the mid-1970s and early 1980s [1][2][3]. Since that time, collisions of lowenergy (eV) organic ions with surfaces within the tandem mass spectrometer have been valuable for analyzing surface composition, characterizing reactions between organic projectile ions and surface adsorbates, chemically modifying surfaces, and determining projectile ion structure. A major motivation for development of SID is that energy transfer to ionic projectiles can be improved by increasing the mass of the collision target. The total available energy for transfer into the internal modes of the projectile ion is defined by the center-of-mass energy (E COM ) described by eq 1:where E LAB is the laboratory collision energy and M ION and M N are the masses of the projectile ion and neutral, respectively. Energy transfer in CID is limited by the mass of the collision partner, typically inert gases such as helium, argon, or xenon. In SID, if one assumes that the surface is an infinitely large collision partner, E COM becomes independent of mass and approaches the laboratory collision energy. The assumption that the entire surface can be viewed as the collision partner is not always valid, however, and there are instances where the mass of the terminal groups on the surface influence the amount of energy transfer [4,5]. Nonetheless, the use of a massive surface target should, in theory, provide ...