FMR arises from the precessional motion of the (usually quite large) magnetization  of a ferromagnetic material in an external magnetic field .  The magnetic field exerts a torque on the sample magnetization which causes the magnetic moments in the sample to precess.  The precession frequency of the magnetization depends on the orientation of the material, the strength of the magnetic field, as well as the macroscopic magnetization of the sample; the effective precession frequency of the ferromagnet is much lower in value from the precession frequency observed for free electrons in EPR. Moreover, linewidths of absorption peaks can be greatly affected both by dipolar-narrowing and exchange-broadening (quantum) effects. Furthermore, not all absorption peaks observed in FMR are caused by the precession of the magnetic moments of electrons in the ferromagnet. Thus, the theoretical analysis of FMR spectra is far more complex than that of EPR or NMR spectra.
The basic setup for an FMR experiment is a microwave resonant cavity with an electromagnet.  The resonant cavity is fixed at a frequency in the super high frequency band.  A detector is placed at the end of the cavity to detect the microwaves. The magnetic sample is placed between the poles of the electromagnet and the magnetic field is swept while the resonant absorption intensity of the microwaves is detected.  When the magnetization precession frequency and the resonant cavity frequency are the same, absorption increases sharply which is indicated by a decrease in the intensity at the detector.
Furthermore, the resonant absorption of microwave energy causes local heating of the ferromagnet. In samples with local magnetic parameters varying on the nanometer scale this effect is used for spatial dependent spectroscopy investigations.
The resonant frequency of a film with parallel applied external field  is given by the Kittel formula:
where  is the magnetization of the ferromagnet and  is the gyromagnetic ratio.