With the evolution of wireless communication, varactors can play an important role in enabling adaptive transceivers as well as phase-diversity systems. This thesis presents various varactor diode-based circuit topologies that facilitate RF adaptivity. The proposed varactor configurations can act as variable capacitors with high tuning range, low losses and ultra-low distortion, while being continuously tunable and facilitating fast modulation. Making use of these special components, we dealt with various RF applications that can benefit from their unique features, like power and impedance control in the mobile systems and multiple-standard modulators for transceivers and phase diversity systems. Chapter 1 provides an overview of challenges associated with the evolution of wireless communication. Through several case studies, it has been addressed how linear variable reactors (varactors) can enable RF reconfigurability for future telecommunication systems. The challenges on varactors for these applications are brought out, which suggests an urgent need for high-performance varactors. This chapter ends with a descriptive and flow-graph-like outline of the thesis. Chapter 2 presents an overview of the state-of-the-art tunable elements such as BST varactors, MEMS based switches and varactors, and currently available semiconductor switches and varactors. Their advantages and drawbacks are extensively discussed. These surveys clarify the motivation and goal of this thesis, and can at the same time be used as a reference to place this research with respect to the existing literature. To overcome the limitations of currently available tunable elements, Chapter 3 deals with the theory of two novel extremely linear varactor diode configurations with complementary linearity properties in a single varactor diode technology. Both varactor configurations use anti-series varactor diode configurations, where the diodes share the same exponential C(VR) depletion capacitance relation. However, the proposed structures differ in their harmonic terminations and varactor area ratios, resulting in a fundamentally different linearity behavior versus tone spacing. It is this feature that makes it possible to address different requirements of transmit and receive chains in one single technology. In Chapter 4, all varactor configurations, aiming for the cancellation of third-order intermodulation distortion, are summarized and their performance are compared. It is shown that the unique feature of the narrow tone-spacing varactor stack, compared to other “infinite” impedance center-tapped varactor stacks and MEMS varactors, is its high modulation frequency for operation and high linearity for signals with low tone spacing, making it perfectly suitable for many dynamic RF modulation applications. The wide tone-spacing varactor stack, which can be implemented in the same process technology as the narrow tone-spacing varactor stack, offers a complementary linearity behavior in terms of tone spacing and it can be regarded as a bonus, provided that the use of the narrow tone-spacing varactor stack is compulsory. In addition, their exponential C(VR) relationship generally yields larger tuning range compared to the uniformly doped varactors, i.e., the distortion-free varactor stack. When the multi-stack topology is used to further improve the IM5 dominated linearity and power handling capability, it turns out that this stacking yields a linearity improvement that is the double of what is generally found for IM3 dominated devices. The system-level responses of the different varactor configurations are investigated under different bandwidth or data-rate conditions. It reveals that the narrow tone-spacing varactor stack is suitable for both moderate and high data-rate applications, while varactor configurations with linearity limitations at low tone spacings, like the distortion-free varactor stack, may raise some in-band distortion when the bandwidth under consideration is relatively small. Chapter 5 discusses the technology implementation issues and provides the experimental verification of the proposed varactor configurations. The measurement results provide the experimental evidence for the predicted IM3 cancellation, as well as, for the complementary linearity behavior of the narrow tone-spacing varactor stack and wide tone-spacing varactor stack. Their usability in practical circuit conditions was demonstrated through source-pull simulations and measurements, illustrating that high linearity can be maintained in all cases. The multi-stack topology is used to further reduce the IM5 dominated nonlinearity of the narrow tone-spacing varactor stack, yielding a record high linearity for continuously tunable capacitances. Using the ability to adjust the C-VR relationship through the doping profile, the desired capacitance control range and related control voltage are achieved for various practical applications. In particular, the measured data of Skyworks’ pre-production GaAs varactors represent the current state-of-the-art in tuning range, linearity and quality factor among all existing continuously tunable elements. As two application examples of the novel varactors, the adaptive matching networks for mobile handsets are demonstrated in Chapter 6, while a phase shifter and amplitude modulator are given in Chapter 7. The demonstrated adaptive matching networks in Chapter 6 are focused on the efficiency enhancement of the power amplifier in the presence of antenna mismatch. Making use of a varactor-based approach, the resulting networks are capable of dynamically correcting the antenna mismatch with the VSWR of 10 over the whole Smith-chart. For all these conditions, an optimum loading for a power level between 0.5 W and 1 W is offered to the power amplifier stage along with a relatively high operating power gain. The proposed “whole Smith-chart” solution will ease the design of the RF frontend and antennas, yielding a significant reduction in the time-to-market of mobile phones. As another application example, given in Chapter 7, ultra linear low-loss varactors are applied for the implementation of amplitude and phase modulators, which can be used in phase diversity systems. The designed structures allow rapid amplitude and phase modulation with a very low distortion. These components can not only improve the performance of existing RF systems, like phased-array antennas and active load-pull system, but also facilitate other new circuit implementations or RF applications. As a demonstration, a novel polar modulator is proposed that can considerably simplify the structure of the traditional transmitter architecture, while being capable of generating the complex signals, which are typically in use in wireless communication systems. Chapter 8 presents the conclusions and recommendations of this research. The most important conclusion is that the linearization techniques proposed in this thesis has enabled the implementation of ultra linear low-loss varactors. Making use of these varactors, various adaptive circuits can be designed for adaptive RF systems.