Recent advances in nanofabrication have produced increased interest in the scattering and emission properties of nanoparticles and confined optical sources. In this context, the new area of optical antennas aims at achieving the bridge between nanoscale optical sources, light-matter interaction and far-field radiation. In some senses the operation of optical antennas is analogous to that of conventional radio-frequency (RF) antennas, which transfer confined electrical signals into radiated propagating electromagnetic waves. Even if their functionality is similar, optical and RF antennas show drastic differences in physics, material properties and excitation features. In this paper, we review some of the concepts of optical antenna theory, design and applications and we highlight how the optical nanocircuit paradigm may be applied to achieve optical impedance matching, loading and tuning in the optical domain. We also provide an overview of how these concepts may have relevant applications in a variety of fields of optics, such as sensing, optical communications and nonlinear nanodevices.