There is considerable interest in imaging molecules adsorbed on surfaces with a scanning tunneling microscope (STM), due to the potentially high resolution available and the enormous potential of such studies in chemical directions. The results observed on adsorbed molecules on conducting surfaces with the STM are reviewed. From these results, it is clear that the mechanism of tunneling through the molecule as well as the parameters which provide the contrast in the STM images are not fully understood. We then show that the molecules under the tip of the STM are subject to significant forces : both van der Waals forces and electrostatic ones. These forces are different from one molecule to another and from one functional group (in the same molecule) to another. Most measurements were done in room temperature. Therefore both molecules must move with respect to the others and also motions of functional groups within the molecule must occur : The activation energies of these processes are of the same order of magnitude as the tip-molecule interactions. We try to evaluate quantitatively the extent to which these motions occur and their effect on the observed contrast. Most studies so far claimed that the observed STM image is related to the HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) of the molecules. The HOMO and the LUMO were calculated for some molecules which were studied experimentally, and simulated images in a gray scale representation were calculated and were compared with the STM images. The lateral solution of the simulated image was artificially reduced to 2.5 angstrom 5(which is the resolution of the STM). The simulated images were calculated for the different possible conformations of the molecule on the surface. The STM image is probably an average of the different states at the different conformations.