I will review recent results of stratified fluids, illustrating the fascinating properties of internal gravity waves. I will in particular discuss three different physical mechanisms: internal wave reflection, mean flow generation and parametric subharmonic instabilities. I will mostly emphasize theoretical results but also discuss related experimental observations.

G. Bordes, A. Venaille, S. Joubaud, P. Odier, T. Dauxois, Physics of Fluids 24, 086602 (2012) -
Experimental observation of a strong mean flow induced by internal gravity waves.

B. Bourget, H. Scolan, T. Dauxois, M. Le Bars, P. Odier, S. Joubaud, Journal of Fluid Mechanics 759, 739 (2014) -
Finite-size effects in parametric subharmonic instability.

H.H. Karimi, T.R. Akylas, Journal of Fluid Mechanics 757, 381 (2014) -
Parametric subharmonic instability of internal waves: locally confined beams versus monochoromatic wavettrains.

T. Kataoka, T.R. Akylas, Journal of Fluid Mechanics 769, 621 (2015) -
On three-dimensional internal gravity wave beams and induced large-scale mean flows.