This chapter describes some important properties of internal tides. The diurnal and semidiurnal spectral peaks related to the internal tides consist of separate waves with close frequencies. Internal tides are modulated due to the spring-neap variability of the barotropic tide that generates them. Strong internal tides usually have mode structure, which is considered here on the basis of field measurements. Fluctuations of currents at a semidiurnal frequency consist of the currents induced by the barotropic tide and internal tide. The methods for separating them are considered. The beam propagation of internal tides near the generation regions of submarine slopes is considered. The long distance propagation of internal tides and decay of their energy are analyzed based on observations and modeling. Mixing induced by internal tides influences the propagation of Antarctic Bottom Water to the north in the Atlantic Ocean.