The control of gene expression in higher eukaryotes has developed into an invaluable tool for both the design of gene therapy strategies and the study of gene function. Application of gene therapy to a number of diseases will require that therapeutic protein expression level is adjusted to prevent toxicity and to ensure biological efficacy. Amongst currently available chimeric regulatable systems, those derived from the tetracycline operon of Escherichia coli are by far the best documented and the most frequently used. This review will describe the various tetracycline systems. Advantages and drawbacks for in vitro studies and in vivo applications will be presented, as well as the development of viral vectors and gene therapy applications. Tetracycline regulatable expression systems progressively turned out to be effective tools for a broad range of investigations. These include studies of gene function in transgenic animals, analysis of complex and multi-staged biological processes including embryogenesis and cancer, and the generation of animal models of human disorders aimed at understanding the cause and progression of diseases. Strategies developed for these various purposes will be discussed and examples will be given, preferentially with respect to the central nervous system.