being reached, recent studies indicate that this is not an issue for discs a few times more massive than the minimum mass solar nebula, or with supersolar metallicities [e.g. @Chiang2008; @Leeetal2010a; @Leeetal2010b]. Moreover, @Youdin2011 showed that inclusion of gas drag could alleviate the need to actually reach the Roche density. However, these mechanisms assume a low level of global turbulence. While the magnetorotational instability (@BalbusHawley1998) may be suppressed in the so-called dead zone [@Gammie1996], fluctuations arising from forced density waves or other instabilities could still be present [@FlemingStone2003; @LesurPapaloizou2010; @Latteretal2010]. Concentrating the dust is problematic.

However, interesting effects already occur as soon as the dust-to-gas ratio is of order unity, say around the midplane, or as a result of turbulent concentration [e.g. @Cuzzietal2003]. This is because the dynamical back-reaction of the dust on the gas is then important, while self-gravity itself may still be negligible. In fact, @YoudinGoodman2005 (hereafter YG05) discovered that the interpenetration of dust and gas in a Keplerian disc was linearly overstable: a process that they named the *streaming instability*, a convention we shall henceforth adopt. The dust particles can behave collectively via their interaction with the gas, leading to disordered flows that facilitate grain clumping—as witnessed 