Contact to single atoms and molecules with the tip of a scanning tunnelling microscope

Experiments using the tip of a scanning tunnelling microscope to contact atoms and molecules adsorbed on surfaces are reviewed. Conductance quantization upon forming or breaking a contact between the tip and surfaces as well as between the tip and specifically chosen atoms and molecular orbitals is addressed. Imaging the contact area prior to and after contact measurements allows one to monitor the status of the contacted object as well as that of the contacting electrodes. Spectroscopy with the tip in contact with individual atoms or molecules reveals the reproducibility of and control over such experiments today. (topical review)