A dynamic probe detection system (29,32) is for use with a scanning probe microscope of the type that includes a probe (18) that is moved repeatedly towards and away from a sample surface. As a sample surface is scanned, an interferometer (88) generates an output height signal indicative of a path difference between light reflected from the probe (80a,80b,80c) and a height reference beam. Signal processing apparatus monitors the height signal and derives a measurement for each oscillation cycle that is indicative of the height of the probe. This enables extraction of a measurement that represents the height of the sample, without recourse to averaging or filtering, that may be used to form an image of the sample. The detection system may also include a feedback mechanism that is operable to maintain the average value of a feedback parameter at a set level.