In this book dielectric and microwave investigations on different nematic liquid crystalline systems are presented. New knowledge about the influence of the chemical structure of molecules on the microwave performance was obtained. This information was used for the preparation of new, optimized nematic mixtures with high values of tunability and low dielectric losses in the microwave region. These new mixtures can be used for producing new devices like phase shifters or varactors. The dielectric measurements of investigated mixtures show that there is no direct influence of dielectric properties in the low frequency region on the GHz region. The influence of different structural fragments of molecules on their properties in the dielectric and microwave region was studied.