The constantly growing traffic of digital data makes a more effective use of fibre optic cables necessary, e.g. by using several data channels having different light wavelengths. This requires a switching of data from one channel to another, i.e. a conversion between different light wavelengths. One possible solution is the use of integrated-optic waveguide devices in the nonlinear material lithium niobate (LiNbO3). Lithium niobate shows the so-called photorefractive effect. This effect is detrimental to nonlinear optic interactions, decreasing their efficiencies or inhibiting them at higher powers. Different waveguide technologies promise a reduction of the photorefractive sensitivity. For this work, titanium-diffused waveguides were fabricated in MgO-doped congruent and stoichiometric material. Zinc-diffused waveguides were fabricated in undoped LiNbO3, and annealed proton exchanged waveguides in undoped and MgO-doped LiNbO3. Their optical properties were investigated. The fabrication of periodically inverted ferroelectric domain structures was studied. Experiments concerning second harmonic generation, to assess the nonlinear performance, and wavelength conversion were conducted successfully. The photorefractive sensitivity was studied at half of the telecommunication wavelength. No reduction of the photorefraction could be detected in titanium-diffused waveguides. Zinc-diffused and especially proton exchanged waveguides showed a significantly reduced photorefractive effect. Those kinds of waveguides thus are suitable choices for wavelength conversion at high optical powers.