Time-domain image-current or related ion signals that are converted by Fourier transformation into a frequency-domain mass spectrum constitute Fourier transform mass spectrometry (FTMS). The frequency of ion motion—cyclotron frequency in an ICR cell or axial oscillation frequency in an electrostatic (Orbitrap) trap—is a precise function of mass-to-charge ratio, so that long, coherent transients yield high resolving power ^[1]. Space charge shifts those frequencies and must be accounted for in mass calibration ^[2]. Collisional or electron-based activation of trapped multiply charged ions can be performed inside the same analyzer, linking the FTMS measurement to tandem structural analysis ^[3]. Conceptual boundaries exclude scanning beam instruments (magnetic sector, quadrupole) and time-of-flight analyzers that do not record a Fourier-transformed transient, and they include both FT-ICR and Orbitrap implementations as members of the FTMS class ^[4]. Essential features are ion trapping that supports periodic motion, broadband detection of that motion, and a Fourier-transform conversion from time to mass-to-charge ratio.