We present optical slot antennas and their applications to photonic devices. We show that metallic nanoslots have the properties of a slot antenna by measuring the transmission spectra and far-field radiation patterns and then prove that they can be physically regarded as magnetic dipoles in the optical region. Additionally, we can generate directional radiations from optical slot antennas by adopting the geometry of radiofrequency Yagi-Uda antenna and properly adding auxiliary elements called reflectors and directors to a single slot antenna. We present two cases as the applications of optical slot antennas. One is the integration of slot antennas to plasmonic waveguides. This combination can be used as a basic unit for optical interconnection to free space and plasmonic via in multilayered plasmonic structures. The other is the integration of slot antennas to the electrode of light-emitting diodes (LEDs). Using slot antennas, we can control the polarization and direction of emissions from LEDs. Besides the above-mentioned two cases, we expect that optical slot antennas have possible applications to various photonic devices and can be essential elements in future integrated photonic circuits with nanometer scales.