In FM AFM, the cantilever acts as the oscillator in an active feedback circuit. The resonant frequency of the lever changes due to the force interaction between the cantilever tip and sample. Changes in the resonant frequency are detected by an FM demodulator. The feedback is used to keep the lever oscillating at its resonant frequency and at constant amplitude. This technique facilitates the use of high Q levers without restricting the bandwidth or the dynamic range. FM AFM is usually used in ultra-high vacuum but we wish to use the highly sensitive technique in liquid. This has been achieved by combining FM-AFM with direct magnetic activation of the lever. We have recently deduced simple equations that enable the interaction force and energy to be determined directly from the measured frequency shift (see extracting force from measured frequency shifts in FM detection).