This thesis describes the research carried out to develop several novel periodically poled lithium niobate (PPLN) devices. These devices exploit the ability to invert micro-domains of spontaneous polarisation in ferroelectrics such as lithium niobate. The fabrication of PPLN devices is described and extensive studies into factors influencing the poling quality are presented. In particular a comparison of material properties of unprocessed lithium niobate material from a range of different suppliers is carried out. Several novel PPLN devices are reviewed and two main devices are investigated - an electro-optically controlled Bragg grating modulator for laser beam switching and modulation and a titanium indiffused waveguide in PPLN for frequency conversion. The design, fabrication and operation of the electro-optic Bragg modulators is described and results for the first infrared operation at 1064nm of such a device are presented. Several discrepancies are seen between experimental results, both in this thesis and previously published results, and a theoretical model based on Kogelnick?s coupled wave analysis. These anomalies are further investigated at visible operation (633nm and 488nm) and solutions and methods for alleviating the discrepancies are presented. Work on titanium indiffused channel waveguides in PPLN is reported, including a study into fabrication issues and the demonstration of second harmonic generation of 416nm in such a device.