Hysteretic and nonlinear dielectric behaviour in ferroelectric ceramics has been of interest since 1950s, when these materials found application in various electronic devices. Presently, these phenomena concern with important areas of science, technology and engineering. In particular, nonlinearity and hysteresis are the key factors in performance, precision and accuracy of modern devices. Many theoretical and experimental studies have been aimed at understanding the origins of hysteresis and nonlinearity in ferroelectrics. Nowadays, there are several models that describe major contributions to nonlinearity and hysteresis on phenomenological, microscopical or statistical levels. These models have a limited area of applicability due to the complexity of physical processes occurring in real materials. Empirically, hysteresis and nonlinearity in ferroelectrics can be controlled by softening and hardening of the material. This is the case of most widely used ferroelectric, lead zirconate titanate (PZT). The soft compositions possess large electro-mechanical coefficients but also large hysteresis and nonlinearity while the opposite is true for the hard compositions. After fifty years since introduction of these materials, the mechanisms of softening and hardening remain poorly understood. The present study is aimed at a better understanding of the processes leading to hardening and softening of Pb(Zr,Ti)O3 ceramics in order to verify the key principles required for a more universal physical model of hysteresis and nonlinearity. Based on the present state of knowledge, such model should consider domain wall contribution to nonlinear and hysteretic polarization response and at the same time account for hardening and softening of the ferroelectric. For this purpose the well known lead zirconate titanate (PZT) ceramics doped with various concentrations of niobium (soft materials) or iron (hard materials) are chosen as a prototype of the ferroelectric system. The starting hypothesis of the thesis' approach is that the softening and hardening are a result of electrostatic arrangement of charged defects in the ceramic bulk: the hard materials are characterized by the ordered and the soft by disordered defects. The thesis then develops in detail the idea that hardening-softening transitions in a ferroelectric system may occur under the influence of (i) dopants, depending on their type and concentration, (ii) a cyclically applied electric field, (iii) a thermal treatment, and (iv) time. The transition from microscopic order to microscopic disorder is confirmed experimentally using carefully analyzed phenomenological parameters of the macroscopic hysteresis and nonlinearity. Among the nonlinear and hysteretic parameters characterizing the polarization response of a ferroelectric material, some (e.g., third harmonic of polarization) are shown to be particularly sensitive to the softness and hardness of ferroelectric system and thus may serve as the characteristics of ferroelectric hardening-softening transitions. Contribution of domain walls to hysteresis and nonlinearity is analyzed in terms of domain wall energy potential and degree of ordering of pinning centres. It is shown that two existing models characterizing hard (V-potential) and soft (random potential) materials are ideal, limiting cases and that some real materials are described by an intermediary case, which can evolve with time and under influence of external factors. The dielectric characterization performed at wide range of frequencies has revealed an increase of the apparent frequency dispersion of the dielectric permittivity with the transition from the hard to soft state in PZT ceramics. The investigation of dielectric response over a wide temperature range has revealed the profound presence of hopping conductivity in iron doped PZT ceramics below the Curie temperatures and its absence in niobium doped PZT ceramics. The role of hopping charged species in ferroelectric hardening – softening transitions is analyzed and discussed. The thesis is organized in the following way. A brief introduction (Chapter 1) and a literature review of the theoretical description of domain wall contribution to dielectric nonlinearity and hysteresis in ferroelectrics (Chapter 2) is followed by the thesis outline and discussion of a unified model of hysteresis and nonlinearity in ferroelectrics with ordered and disordered states of domain wall pinning centres (Chapter 3). Processing of ceramics is described in Chapter 4 and mathematical and experimental background for the dielectric spectroscopy study in Chapter 5. The results and discussion of detailed experimental studies of polarization response in ferroelectric PZT ceramics under subswitching and switching conditions are given in Chapters 6 and 7. The summary of the main results and conclusions are given in the last thesis section.