Atomic force microscopy (AFM) is a unique tool for the characterisation of functional biomolecules and biomolecular complexes. We have refined AFM technology to achieve highest resolution in liquid environment, as demonstrated by imaging solid-liquid interfaces at atomic resolution and by resolving different structural conformations of the DNA double helix. Going beyond surface structure alone, we can also map electrostatic
properties of individual biomolecules, and probe nanomechanical properties inside biomolecular machines. The latter point is exemplified by our measurements on the nuclear pore complex, where we determine the nature of its permeability barrier and interpret our results using polymer physics.