SUMMARYThe scan speed limit of atomic force microscopes has been calculated. It is determined by the spring constant of the cantileverk, its effective massm, the damping constantDof the cantilever in the surrounding medium and the stiffness of the sample. Techniques to measurek, k/mandD/mare described. In liquids the damping constant and the effective mass of the cantilever increase. A consequence of this is that the transfer function always depends on the scan speed when imaging in liquids. The practical scan speed limit for atomic resolution in vacuum is 0·1 μm/s while in water it increases to about 2 μm/s due to the additional damping of cantilever movements. Sample stiffness or damping of cantilever movements by the sample increase these limits. For soft biological materials imaged in water at a desired resolution of 1 nm the scan speed should not exceed 2 μ