In depth study of the crystal structures of piezoelectric materials as a function of temperature, pressure and composition allows for the design and optimization of such materials and defines the conditions of their use in technological applications. Results from studies on two classes of piezoelectric materials are described, the α-quartz group and the ferroelectric perovskite group. The structures of α-quartz-type germanium dioxide and iron phosphate were refined at high temperatures by the Rietveld method using time-of-flight neutron powder diffraction data. The α-β phase transition occurs at 980 K in FePO4, whereas for GeO2, no β phase is observed. The intertetrahedral bridging angle θ and the tilt angle δ in GeO2 exhibit thermal stabilities that are significantly greater than α-quartz. The temperature dependence of these angles is found to be a function of the initial structural distortion in α-quartz homeotypes with the notable exception of α-quartz-type FePO4, which appears to be dynamically unstable. The stability of α-quartz and α-quartz-type germanium dioxide was investigated at high pressure by x-ray powder diffraction. New six-fold coordinated forms were found in both materials. The important, perovskite-type, piezoelectric material PbZr0.52Ti0.48O3 was studied up to 18 GPa by angle-dispersive, x-ray diffraction using an imaging plate and by Raman spectroscopy. A novel phase transition was found in this system at close to 5 GPa. Whereas the x-ray diffraction data indicated no deviation from cubic symmetry above this pressure, a strong Raman signal was present in this phase, which is similar to those observed for ferroelectric relaxors.