Protein degradation has been identified as a major mechanism for the regulation of cellular functions. Not surprisingly, its deregulation is implied in almost any pathological condition. This book describes how aged proteins are eliminated during cell metabolism, how cell proliferation is regulated by protein degradation and how its deregulation can contribute to the development of cancer, how protein degradation is modified during normal and abnormal aging, in particular with regard to Alzheimer's disease and other degenerative diseases of the brain and central nervous system. Attempts aiming at correcting these pathologies by interfering with deviations of the normal pathway of protein degradation are also treated.