  Internal gravity waves play a primary role in geophysical fluids: they
contribute significantly to mixing in the ocean and they redistribute energy
and momentum in the middle atmosphere. Until recently, most studies were
focused on plane wave solutions. However, these solutions are not a
satisfactory description of most geophysical manifestations of internal gravity
waves, and it is now recognized that internal wave beams with a confined
profile are ubiquitous in the geophysical context.
  We will discuss the reason for the ubiquity of wave beams in stratified
fluids, related to the fact that they are solutions of the nonlinear governing
equations. We will focus more specifically on situations with a constant
buoyancy frequency. Moreover, in light of recent experimental and analytical
studies of internal gravity beams, it is timely to discuss the two main
mechanisms of instability for those beams. i) The Triadic Resonant Instability
generating two secondary wave beams. ii) The streaming instability
corresponding to the spontaneous generation of a mean flow.
