This talk concentrates on the development of frequency agile circuits based on thin film feroelectric materials. Central to this effort is the use of barium strontium titanate (BST) thin films. BST is a low loss, high dielectric constant non-linear dielectric material. The electric field dependence of BST permittivity allows fabrication of tunable RF and microwave dievices and components. Some recent results on the fabrication and measurement of high Q varactors and tunable filters will be presented. Other applications of BST based varactors for the design of adaptive matching circuits and linearized power amplifiers will also be discussed.