We performed scanning tunneling microscopy (STM) experiments on mono-or sub-monolayer deposited porphyrin molecules on metal substrates. We observed substitution-dependent structure changes of the molecules. Obtaining STM images of the structure and arrangement of functional molecules would contribute valuable information towards creation of nano-molecular-devices. scanning probe microscopy (SPM) shows the greatest potential for reaching this goal, at present. STM is able to obtain images with nano-meter resolution of molecules on metal surfaces. These high-resolution images reveal the molecular structure and arrangement of the molecules on the surface and contain information about the molecule-substrate interaction and the molecule-molecule interaction. STM will be useful for the assembly of nano-devices made from molecules on a substrate. (C) 2003 Elsevier B.V. All rights reserved.