  The study of rotation and activity in low-mass stars or brown dwarfs of
spectral classes M and L has seen enormous progress during the last years. I
summarize the results from different works that measured activity, rotation,
and sometimes magnetic fields. The generation of magnetic activity seems to be
unchanged at the threshold to completely convective stars, i.e. no change in
the efficiency of the magnetic dynamos is observed. On the other hand, a sudden
change in the strength of rotational braking appears at the threshold mass to
full convection, and strong evidence exists for rotational braking weakening
with lower mass. A probable explanation is that the field topology changes from
dipolar to small scale structure as the objects become fully convective.
