We present ab initio simulations of AFM image formation in the non-contact regime for prototypical reactive semiconductor and metal surfaces: InP(1 1 0)- 1 x 1 and Cu(0 0 1). For the reactive surface the effect of tip morphology of the tip apex was also studied. The nature of the tip apex alters the local tip reactivity and can lead to reversal of the apparent AFM surface corrugation. We find that for both semiconductor and metal surfaces the atomic resolution is primarily mediated by a strong chemical-type of interaction between the tip and the surface. This allows for a unified interpretation of the tip-surface interactions in the non-contact AFM microscopy. (C) 2002 Elsevier Science B.V. All rights reserved.