Images of DNA obtained in a scanning tunneling microscope operating under ultra‐high vacuum are described, and a contrast mechanism for STM imaging of weakly‐bound adsorbates is proposed. The capability of the STM for atom‐scale imaging of DNA is demonstrated through comparison of STM data with the accepted structure of A‐DNA derived from x‐ray crystallography; seven different structural elements of the molecule are identified and used to support the data interpretation. The contrast mechanism proposed invokes the Pauli‐principle based interference of the adsorbate with the substrate wavefunction vacuum tail. The adsorbate wavefunctions themselves lie far from the Fermi level and are unlikely to support electron conduction; their interaction with substrate wavefunction tail, however, enhances the tip‐substrate electron tunneling probability at the adsorption site. The suggested mechanism is supported with ab initio calculations. The paper summarizes a talk presented at the Engineering Foundation conference ‘‘Scanned Probe Microscopies: STM and Beyond.’’