The recent development of scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) has allowed the electronic and atomic structure of interfaces to be probed with unprecedented spatial resolution. Although both STM and TS are now commonly used in investigations of metal and semiconductor surfaces, these techniques can also be applied in chemical investigations of molecular species and molecular films, frequently providing details of their orientation, density, and electronic properties. In this review, the instrumentation and fundamental principles underlying STM and TS are presented and recent examples of STM/TS investigations of molecular interfaces are described. Emphasis is placed on investigations of molecularly thin films of electroactive transition metal complexes, e.g., [(Ru(terpyridine))2(tetrapyridylpyrazine)]-(PF6)4and protoporphyrin(IX)-Fe(III)Cl, deposited on metal and graphite electrodes. In addition, studies of quasi-crystalline molecular films, silver halide films, and surface defects are also presented to demonstrate the broad range of chemical systems that can be explored by these techniques.