KTiOPO4 (KTP) and Rb-doped KTP (RKTP) are two of the most attractive nonlinear opticalmaterials for engineering of periodically poled domain structures, commonly used as frequencyconversiondevices for laser radiation via the quasi-phase matching (QPM) technique. Thesematerials have excellent non-linearity, wide transparency windows and high resistance to opticaldamage. Furthermore their large domain-velocity anisotropy allows the fabrication of highaspect-ratio domain structures, needed for many QPM applications. To create highly efficientdevices, precise control over the structure uniformity and duty-cycle is required. Constantimprovement of the domain engineering techniques has allowed pushing the limits of theachievable domain aspect-ratio. For this development to continue, a deeper understanding of theformation dynamics and stability of the domain gratings is of utmost importance. As the domainsizein nanostructured devices decreases, the density of the domains walls (DWs) increases andtheir properties are ever more important for device performance. Indeed, more knowledge on thedomain wall properties, and the means to engineer them, could enable new applicationsexploiting these properties.This thesis presents studies on domain wall properties and dynamics in KTP and RKTP. Thesub-millisecond dynamics of grating formation in RKTP under an applied electric field has beenstudied in the high-field regime using online second harmonic generation. The effects ofdifferent pulse shapes were compared and single triangular pulses were found to be superior interms of the resulting grating quality.The high-temperature stability of domain gratings was investigated. The domain wall motioninduced by annealing was shown to be highly anisotropic along the a- and b-crystal axes, anddependent on the period of the grating period.The local charge transportation at the domains and domain walls in KTP was characterized usingatomic force microscopy, demonstrating a fourfold increase of conductivity at the walls.Voltage-cycling measurements revealed memristive-like characteristics, attributed to the effectof ionic motion and local charge accumulation. The enhanced conductivity of charged domainwalls was used as an imaging tool, to study domain wall dynamics while inducing motionthrough the application of an external field.Finally, the interplay between ionic motion, spontaneous polarization and polarization reversalwas investigated, showing direct evidence of elastic modulus modification during localpolarization switching.