This thesis focuses on the design and characteristics of different antennas in the optical frequency range. Relevant quantities such as intensity enhancement or directive gain are calculated by several semi-analytical and numerical methods and are optimized via automatic optimization algorithms, which might lead to new or unintuitive structures. In principle there are two different antenna types: Receiving antennas, which focus the energy of plane waves on a very small area, and sending antennas, which transform the nearly isotropic emission of a dipole source into a desired radiation pattern, mainly a beam into the desired direction. First, already existing, receiving antennas, which are characterized by a high field enhancement in a very small area, are considered as the start of an optimization. It is shown here that some geometries in arrays exhibit resonances that can be traced back to collective array modes. As a result, not only the maximum intensity enhancement is drastically increased, but these modes also exhibit a higher lifetime, so that more energy from the plane wave can be used. After that a free optimization is discussed, resulting in a new structure, whose performance is significantly increased compared to already existing antennas. Subsequently, transmitting antennas, which are characterized by a high directive gain, are discussed. In this thesis it is shown that suitable, dielectric structures that correspond to a weakly guiding waveguide, show a remarkable performance. Although such antennas are larger, they show a significant improvement in the directive gain and are robust with respect to fabrication tolerances, which has been demonstrated experimentally. In addition, a dielectric antenna is proposed, which is significantly smaller than the wavelength of the emitted light, and with which various desired radiation patterns can be obtained.