This thesis concentrates on the development of advanced large-signal measurement and characterization tools to support technology development, model extraction and validation, and power amplifier (PA) designs that address the newly introduced third and fourth generation (3G and 4G) wideband communication standards. By exploiting an innovative mixed-signal approach, the measurement systems developed within this thesis work extend the limits of the current state-of-the-art large-signal characterization in terms of bandwidth, power range, speed and functionality. As described in Chapter 1 these activities are needed to address the demands that follow from the evergrowing data transfer rates in modern telecommunication systems. Here the introduction of the new 3G and 4G communication standards, which make use of higher bandwidths and peak-to-average ratios, in combination with the necessity to reduce the power consumption of mobile networks, put very stringent demands on the power amplifier in the wireless transmitter. Since the PA is one of the dominant sources of energy consumption in the wireless network, it needs to be very efficient and linear over a wide frequency range and over a wide power span. Therefore, to accomplish this difficult mission, the PA designer has to rely on either an accurate nonlinear model of the active device to perform the design in a circuit simulator, or on device data resulting from load-pull measurements, which can accurately characterize the transistor performance parameters as a function of the load and source impedances at all frequencies of interest. In order to support the development of compact device models which include self-heating and trapping effects, isothermal measurement systems are required. Chapter 2 introduces the theory and the requirements for pulsed-RF and pulsed-DC measurements. Moreover, a new isothermal measurement system is presented, which provides the ability to measure with DC and RF pulses as short as 200 ns, while featuring a very high dynamic range (? 85 dB) under pulsed-RF conditions, which is independent on the duty-cycle. The system performance is discussed in detail through a set of benchmarks, and some examples on isothermal active device characterization are provided. The second and dominant part of this thesis introduces a revolutionary active harmonic load-pull approach. Load-pull device characterization is fundamental to all activities related to PA design and PA development, from technology development, to model extraction and validation, to the actual power amplifier design. For this reason Chapter 3 reviews conventional passive and active source and load-pull architectures, and discusses their basic limitations, with particular attention to the problems arising when characterizing devices with wideband complex modulated signals. Moreover, the requirements of active load-pull systems to perform high power measurements with complex modulated signals are also explained. To solve the problems of conventional load-pull systems when dealing with wideband modulated signals, a novel active harmonic load-pull system based on a mixed-signal approach is described in detail in Chapter 4. The system developed during this thesis work enables the measurement of active devices up to 120 MHz of modulation bandwidth, and allows arbitrary control of the refection coefficient in this band. Measurement data highlighting the system performance, and measurement results on active devices are presented. To enhance the process of developing new transistor technologies and their application in very efficient and linear PAs, in Chapter 5, a new approach for enabling high-speed multidimensional source and load-pull parameter sweeps is introduced. The method described allows any combination of multiple parameters (e.g., input power and/or fundamental and harmonic load impedance) to be swept, at a very high speed, while maintaining all other parameters (e.g., second harmonic source impedance) accurately controlled to a user-defined value. Moreover, several measurements are reported, with particular emphasis on the high-power capabilities of the system, both in CW as well as under modulated signal excitations. The option to measure time-domain voltage and current waveforms can provide significant insight into the actual device behavior, which benefits to power amplifier design, ruggedness evaluation, as well as to (database) model extraction and validation. In Chapter 6 the basic theory behind the measurement of high frequency time-domain voltage and current waveforms at the device reference planes are discussed, and an extension to the mixed-signal load-pull system described in the previous chapters is presented, with particular attention on the requirements of the calibration device used for the system calibration. Furthermore an approach for time-domain waveform analysis of multi-tone signals which are closely spaced in frequency is introduced. To highlight the most unique capabilities of the mixed-signal load-pull system developed in this thesis, Chapter 7 reports several relevant examples of significant applications. In particular an out-of-band linearity optimization of an HBT device, the characterization of a GaN device for high efficiency PA design, and some very high-power device measurements for base-station applications are described. Chapter 8 finishes the thesis and gives the most important conclusions and the recommendations for future work.