Until the early 1980s, protein degradation was a neglected research area, and scientists were mostly busy deciphering the genetic code and its translation to the proteome. Destruction of cellular proteins was thought to be a scavenger, non-specific and dead-end process. Although it was known that proteins do turn over, the large extent and high specificity of the process, whereby distinct proteins have half-lives that range from a few minutes to several days, was not appreciated. The discovery of the lysosome by Christian de Duve did not change this view significantly, as it was clear that this organelle is involved mostly in the degradation of extracellular proteins, and their proteases cannot be substrate-specific. The discovery of the complex cascade of the ubiquitin pathway revolutionized the field. It is clear now that degradation of cellular proteins via the ubiquitin system is a highly complex, temporally controlled and tightly regulated process that plays major roles in a variety of basic pathways and processes during cell life and death, and in health and disease. The system is involved in targeting many cellular proteins, among them cell cycle regulators, growth- and differentiation-controlling factors, transcriptional activators, cell-surface receptors and ion channels, endoplasmic reticulum proteins, antigenic proteins destined for presentation on class I MHC molecules, and abnormal/misfolded proteins. Consequently, it is involved in regulating many basic cellular processes, such as cell cycle and division, growth and differentiation, signal transduction and transcription, modulation of the secretory and endocytic pathways, the immune and inflammatory responses, and quality control. With the multitude of substrates targeted and the numerous processes involved, it is not surprising that aberrations in the pathway have been implicated in the pathogenesis of many diseases, with certain malignancies and neurodegenerative disorders being ranked among them.