Experiments on quantum transport in a low temperature scanning tunneling microscope provide the possibility to study atomic and molecular contacts with well-defined electrode-geometry at least for one of the contacts, the surface. Here, we show for a number of examples, the strength of transport studies by STM. For single cobalt adatoms, we demonstrate the abrupt change in the width of the Kondo resonance of the cobalt adatom once the contact regime is reached. Differences of the mechanical properties of junctions with cobalt and gold adatoms are discussed. Finally, we show transport measurements and imaging with individual PVBA molecules. Configurations in which we have investigated quantum transport: through a junction consisting of a single gold adatom, a single cobalt adatom, and a PVBA molecule between the tip of an STM and the surface.