The application of in situ atomic-resolution scanning tunneling microscopy (STM) to the elucidation of local structure and potential-induced phase transitions within:ordered adlayers on monocrystalline metal electrodes is discussed, and illustrated primarily for compressed CO adlayers on Pt(111). The value of the additional structural information on adsorbate binding sites : that can be obtained from infrared reflection-absorption spectroscopy(IRAS) as well as from potentiodynamic STM imaging tactics is emphasized. The atomic-level and nanoscale structural alterations wrought upon the Pt(111)/CO adlayer by bismuth predosing are described as a further illustration of the synergetic value of the spatial and surface-bonding information-provided by STM and IRAS. A comparison is made between the potential-dependent adlayer characteristics of the compressed CO adlayers with iodide adlayers on gold and platinum low-index electrodes. Possible reasons for the marked differences in the phase-transition behavior of these nonmetallic adlayers are briefly discussed, along with comparisons with analogous adlayers in ultrahigh vacuum environments.