Abstract I. Introduction 000  II. Linear Multipoles 000 A. Multipole Fields 000 1. Multipole Potentials 000 2. Ion Motion in 2D Multipole Fields 000 3. Ion Motion in Quadrupole Fields 000 4. Ion Motion in Higher Multipole Fields 000 5. Linear Ion Guides, Collision Cells, and Traps 000 6. Collisional Cooling 000 7. Ion Excitation in the Presence of a Collision Gas 000 8. Ion Isolation 000 9. Trap Capacity 000 A. Mass Discrimination Effects 000 III. Linear Traps Combined with Other Mass Analyzers 000 A. Linear Traps Combined with 3D Paul Traps 000 B. Linear Ion Traps Combined with TOF Mass Analyzers 000 C. Linear Traps Combined with FTICR 000  IV. Linear Traps as Mass Spectrometers 000 A. Fourier Transform Mass Spectrometry in a Linear Quadrupole Ion Trap 000 B. Linear Quadrupole Ion Trap Mass Spectrometer with Radial Ion Ejection 000 C. Linear Quadrupole Ion Trap Mass Spectrometer with Axial Ejection 000 V. Summary and Outlook 000 Acknowledgments 000 List of Symbols 000 References 000 Linear ion traps are finding new applications in many areas of mass spectrometry. In a linear ion trap, ions are confined radially by a two‐dimensional (2D) radio frequency (RF) field, and axially by stopping potentials applied to end electrodes. This review focuses on linear ion trap instrumentation. Potentials and ion motion in linear multipole fields and methods of ion trapping, cooling, excitation, and isolation are described. This is followed by a description of various mass discrimination effects that have been reported with linear ion traps. Linear ion traps combined in various ways with three‐dimensional (3D) traps, time‐of‐flight (TOF) mass analyzers, and Fourier transform ion cyclotron resonance mass spectrometers are then given. Linear ion traps can be used as stand alone mass analyzers, and their use for mass analysis by Fourier transforming image currents, by mass selective radial ejection, and by mass selective axial ejection are reviewed. © 2004 Wiley Periodicals, Inc., Mass Spec Rev