Light manipulation via nanoantennas, especially plasmonic nanoantennas, is an exciting new field, which aims to provide the same benefits at optical frequencies as those given by standard antennas in the radio and microwave frequency regimes. While at lower frequencies metals behave as perfect conductors, with negligible field penetration and absorption, at optical frequencies the electromagnetic field is able to excite plasmons which combine the electromagnetic wave with electronic excitations, giving raise to new properties such as high scattering and field confinement. This thesis focuses on understanding the physical principles of plasmonic nanoantennas and calculating their properties analytically, by deriving the solution for the scattering from cuboidal nanoantennas, and computationally, by presenting an improved discrete dipole approximation on cuboidal point lattices. A theory of scattering from anisotropic particles is derived, showing multiple plasmon resonance and different peaks shifts changing the background index, enabling the study of the magneto-optical effect on nanoparticles, with potential applications in nanospectroscopy, light manipulation and optical sensors. Plasmonic nanoparticles are studied numerically in order to enhance light absorption in thin film silicon solar cells and to enhance the anti-reflection coating properties of triple junction III-V quantum well high efficiency solar cells to increase scattering. Finally, plasmonic nanoantennas are also used experimentally to enhance the photoluminescence and decrease the lifetime of silicon quantum dots for light emitting device applications.