Relaxor Pb(Mg1/3Nb2/3)O3(PMN) and its solid solutions with ferroelectric PbTiO3 (PT) are of considerable interest from both the applications and the scientific point of view. In the past, many attempts were made to prepare and study the properties of these material in thin film form. However, due to difficulties in the preparation of pure phase films with high PMN content, there exists very little knowledge on the properties of these important materials. It is the goal of this thesis to prepare PMN and PMN-PT films without second any phases, to study in detail their structure, dielectric and electromechanical properties, and to see how these properties compare with those of the bulk materials. The studies were carried out along three major directions: PMN, 0.9PMN-0.1PT and PT chemical solution precursor syntheses, thin films preparation, dielectric and electromechanical properties characterization and their interpretation. The synthesis of precursors used for the preparation of PMN, 0.9PMN-0.1PT and PT films is reported in detail. Careful control and optimization of the precursors was important to obtain thin films without second any phases. The presence of new compounds and reaction mechanisms were established in the course of developing reproducible and stable precursors solutions. Crystallization of pure PMN and 09PMN-0.1PT films requires high temperatures (∼800°C) and lead excess in the precursor solutions making processing difficult. The correct choice of seeding layers that favor perovskite phase nucleation and growth minimizing substrate instabilities at high temperatures was essential. This approach allowed us to find a narrow processing window that lead to films without pyrochlore and other second any phases. Important PMN and 09PMN-0.1PT processing parameters are reported. It is shown that crystallographic orientation and microstructure could be modified by controlling the perovskite phase nucleation and growth with different seeding layers (TiO2 and PbTiO3). PMN film epitaxial growth on conductive SrTiO3 single crystals allowed an original transmission electron microscopy study. The results showed that PMN in thin film and bulk forms shows the same structural characteristics (morphology and size of polar regions). However, temperature dependence of unit cell parameters of PMN and 09PMN-0.1PT films seems to be qualitatively different from these obtained in bulk materials. PMN and 0.9PMN-0.1PT thin films show typical relaxor characteristics (dielectric permittivity dependence on frequency and temperature, presence of polar regions, superstructure, ferroelectric hysteresis at low temperature) however significant differences exist between films and bulk materials. Most of these differences (low permittivity, presence of the self-polarization) are at least in part related to the properties of the substrate used, as well as the conditions of films preparation. The presented results clearly show the influence of the measuring signal and processing parameters (AC and DC filed amplitude, sintering temperature, substrate quality) on the dielectric response of the films. Due to the small thickness of the films, the non-linearity of the dielectric properties of PMN and 0.9PMN-0.1PT could be studied over a field range never before used in the characterization of bulk samples. The choice of measuring conditions (field strength) can significantly alter the dielectric behavior of the films, and the use of large fields can explain certain results published in the literature. By investigating the AC and DC field dependence of the dielectric permittivity for <111> oriented films it was possible to show that the decrease of the permittivity with the field, at strong fields, is consistent with the model of coalescence of neighboring polar regions. For the first time, the electromechanical properties of PMN and 09PMN-0.1PT films were characterized in details. Longitudinal electrostrictive (M and Q) and induced piezoelectric (d33) coefficients were determined for both compositions. Compared to bulk materials, it seems that thin film properties are reduced selectively. The reduction of dielectric permittivity and, consequently, the electrostrictive coefficient M is large, whereas the value of the Q coefficient is close to that in bulk materials. The preparation, dielectric, electrostrictive and piezoelectric properties of PbTiO3 films, as end member of the (1-x)PMN-(x)PT binary system, were characterized in detail. Stresses in the plane of the film, which are due to the clamping of the film by the substrate, are most likely responsible for the decrease in the temperature of the dielectric permittivity maximum. It was found that clamping of the film by the substrate does not have a strong influence on the value of the electrostrictive coefficients. The contribution of domain-walls displacement to dielectric and piezoelectric properties was investigated in detail and found to be non-negligible.