This book is intended to give an introduction to intermolecular forces from an experimental point of view. Within the last 10 years the interest has turned more and more into an understanding of the weak, but important, int- molecular forces. New experimental techniques have been developed which have helped to gain more insight into this interesting topic. This book is intended as an introduction for graduate students who are familiar with the main concepts of n~olecular spectroscopy. Special emphasis will be laid on the theoretical concepts. After a detailed description of experimental techniques, the results for two prototype systems which have been the subject of several studies in the literature within recent years will be presented. Ar-CO is becoming the most extensively studied van der Waals complex, theoretically and experimentally. Nevertheless, this example shows that even though the theory has greatly improved and has helped us to improve our knowledge of intermolecular forces, even for relatively simple cases the theory car1 still fall short of an accurate description. For a long time (NH3)2 was considered as a prototype for hydrogen bo- ing. However, subsequent experimental and theoretical studies have revealed the mysteries of the obtained spectra and proved that our previous concept of hydrogen bonds was just too naive.