The review is focused on the currently major aspect of 4-hydroxynonenal (HNE) research--studies that combine biological activities of the aldehyde together with the methods of its identification in cells and tissues. Because there were some excellent reviews on HNE published in recent years, starting in 1990 and 1991 with supreme reviews done by Hermann Esterbauer, who discovered the aldehyde, and colleagues from the Institute of Biochemistry in Graz, this article pays most of attention to the most recent articles, published in the last 15 months. Additionally, an overview on the relevance of HNE is given with respect to the research and publication trends in the period of 10 years (1993-2002) according to the data in the Current Contents and Medline data bases. It is obvious that HNE started in 1993 as a "toxic product of lipid peroxidation" and "second toxic messenger of free radicals", to become in 2002 a reliable marker of oxidative stress, a possible causative agent of several diseases (such as Alzheimer's disease), growth modulating factor and a signaling molecule. Novel analytical methods developed suitable pathways for HNE to become a clinically applicable marker of lipid peroxidation on one side and on the other a standardized parameter of food quality control. As it is also present physiologically in various cells and tissues, it is likely that HNE will soon become one of the most attractive factors for those who search for a small and reactive molecular link between genomics and proteomics.