Radiofrequency electric fields that confine ions in a three-dimensional or linear quadrupole trap, with subsequent mass-selective ejection or isolation, define ion-trap spectrometry. Ions of a chosen mass-to-charge window are stored, optionally fragmented by resonant excitation or ion–molecule reactions, and then scanned out to a detector, so that the trap is both an ion store and a mass analyzer ^[1]. Quadrupole ion traps coupled to gas or liquid chromatography therefore provide tandem mass spectra from a single analyzer rather than from a beam-type collision cell ^[2]. Conceptual boundaries exclude magnetic-sector, time-of-flight, and Fourier-transform ICR analyzers, and they exclude ion-mobility spectrometry that separates ions by collision cross section in a drift gas without RF trapping as the mass-analysis principle. Essential features are an RF trapping potential, a means of mass-selective instability or resonance ejection, and, for tandem work, a controlled activation step inside the trap ^[3]. The recorded observable remains the mass-to-charge ratio of precursor and product ions.