Book excerpt: Assigning Structures to Ions in Mass Spectrometry describes the tools currently available for determining gas-phase ion structures. It surveys current experimental methods for ion production and separation as well as those designed to reveal qualitative and quantitative aspects of gas-phase ions. It also examines how and when to apply computational chemistry and theoretical calculations. Selected case studies illustrate specific challenges associated with ion structure assignment and thermochemical problems. Bringing together key results collected over the past four decades, the book contains the data for describing or identifying ions containing C alone and C with H, O, N, S, P, halogens, and small organic cations.