There has been unprecedented growth in wireless technologies in recent years; wireless devices such as cellular telephones and wireless local area network (WLAN) transceivers are becoming ubiquitous. It is now common for a single hardware device, such as a cellular telephone, to be capable of multi-band operation. Implementing a dedicated radio frequency (RF) front-end for each frequency band increases the component count and therefore the cost of the device. Consequently, there is now a requirement to design RF and microwave circuits that can be reconfigured to operate at different frequency bands, as opposed to switching between several fixed-frequency circuits. Barium strontium titanate (BST) thin films show great promise for application in reconfigurable microwave circuits. The material has a high dielectric constant which can be controlled by the application of a quasi-static electric field, combined with relatively low losses at microwave frequencies. Tunable microwave components based on BST-thin films have the potential to replace several fixed components, thereby achieving useful size and cost reductions. This thesis is concerned with the growth and microwave circuit applications of BST thin films on c- and r-plane sapphire substrates. Sapphire is an ideal substrate for microwave integrated circuit fabrication due to its low cost and low loss. Electronically tunable capacitors (varactors) were fabricated by patterning interdigital electrode structures on top of the BST films. High capacitance tunabilities of 56% and 64% were achieved for the films grown on c-plane and r-plane sapphire, respectively, at 40 V bias. A novel electronically tunable 3 dB quadrature hybrid circuit was also developed. Prototypes of this circuit were initially implemented using commercial varactor diodes, in order to validate the design. An integrated version of the coupler was then fabricated using BST varactors on c-plane sapphire. The results achieved demonstrate the potential of sapphire-based BST thin films in practical microwave circuits.