The IQCM implements the simplest possible way of operating a quartz crystal resonator, which is impedance analysis. A network analyzer sweeps the exciting frequency across the resonance. On resonance, the current through the electrodes dramatically increases. Fitting a resonance curve to the conductance, one finds the resonance frequency, f, and the bandwidth, Γ. This information is available on up to 15 harmonics.The analysis rests on the small-load approximation: Frequency and bandwidth are proportional to the real and the imaginary part of the stress-speed ratio (the "load") at the crystal surface. For purely inertial loading (for instance by a thin film), the load is proportional to the mass and the resonator acts as a "quartz crystal microbalance" in the narrow sense of the word. Monolayer sensitivity is easily reached. For more complex samples, the IQCM probes the oscillatory shear stress at the crystal-sample interface. The applicability of the IQCM goes much beyond classical microweighing.