My personal involvement with the problem of hydrophobic interactions (HI) began about ten years ago. At that time I was asked to write a review article on the properties of aqueous solutions of nonpolar solutes. While surveying the literature on this subject I found numerous discussions of the concept of HI. My interest in these interactions increased especially after reading the now classical review of W. Kauzmann (1959), in which the importance of the HI to biochemical processes is stressed. Yet, in spite of having read quite extensively on the various aspects of the subject, I acquired only a very vague idea of what people actually had in mind when referring to HI. In fact, it became quite clear that the term HI was applied by different authors to describe and interpret quite different phenomena occurring in aqueous solutions. Thus, even the most fundamental question of the very definition of the concept of HI remained unanswered. But other questions followed, e. g. : Are HI really a well­ established experimental fact? Is there any relation between HI and the peculiar properties of water? Is the phenomenon really unique to aqueous solutions? Finally, perhaps the most crucial question I sought to answer was whether or not there exists hard evidence that HI are really important -as often claimed-in biological processes.