M stars (low mass main sequence stars, a.k.a. Red Dwarfs) are the most common form of star in the Universe, and they very often host planets. After settling on to the main sequence they also provide a very steady luminosity environment, so if any M star planets can maintain habitable temperatures through retention of a suitable atmosphere, the Universe is surely teeming with life. However, most M star rocky planets seem to be bare rocks. While it is too soon to give up on rocky habitable planets around M stars, attention naturally turns to habitable subNeptunes, since low density subNeptunes which perforce must have atmospheres are not uncommon around M stars. But are any of them habitable? I will review the physics that determines the habitable zone for subNeptunes, focusing on two classes: habitable subNeptunes for which the main greenhouse effect is supplied by H2, and habitable subNeptune water worlds for which the main greenhouse effect is due to CO2. For the former, the essential issue is the stabilisation of convection by compositional buoyancy arising from mixed H2O H2 atmospheres. For the latter, the physics is the same as that determining the “max greenhouse effect” on rocky planets, so I will first review some less appreciated physics of the max GH effect. CO2 supported subNeptune waterworlds raise some very novel and largely unexplored issues of how the deep carbon cycle works on such planets. I will pay particular attention to LHS1140b, which has been inferred indirectly to have an atmosphere, through observations of helium escape. Is LHS1140b a rocky planet, a subNeptune water world, or something in between?