Since the first tetracycline-controlled transcriptional activation system was designed nearly a decade ago, new variants, modifications, and improvements have been steadily added to this powerful set of tools for temporal control of transgene expression in mammalian systems. Tetracycline-based externally regulatable (Tet-based) systems have been successfully used to control the expression of numerous transgenes in cultured cells and in whole organisms, especially in mice. The application of these systems has provided invaluable insights into the function and regulation of a variety of genes under physiological and pathological conditions. Because of the favorable characteristics of the inducing agent doxycycline and the efficiency and effectiveness of the operating mechanism, the Tet-based systems have attracted substantial attention from the transgenic research community and are rapidly gaining popularity. The original tetracycline-controlled transcriptional activator (tTA) is a regulator with tight control of target gene expression and a broad range of inducibility. The reverse tetracycline-controlled transcriptional activator (rtTA) activates the responsive elements only in the presence of doxycycline, giving a convenient control over the target transgene. The recently developed tetracycline-controlled transcriptional silencer (tTS) has been successfully used in cultured cells and in transgenic mice. In combination with rtTA, tTS actively suppresses background expression or "leakiness" without impeding the inducibility of the target gene, providing a true "On/Off" transgenic switch. New variants of Tet-based regulators with improved features are still emerging and the utilities of these systems are constantly being tested.