Very low-mass stars and brown dwarfs are fully convective, a reason to
expect that the solar-like dynamo does not work.  Implications for the
pattern of magnetic activity are expected. Putting X-ray emission in
context with activity signatures in other wavebands (optical and
radio) allows to understand the changes -- if any -- of the coronal
heating mechanism across the fully convective boundary and the
hydrogen burning mass limit. At young age, moreover, stars and brown
dwarfs possess circumstellar disks from which they accrete
matter. Accretion shocks represent potential sites of X-ray emission
next to the coronal plasma. Similarly, accretion contributions are
superposed on the chromospheric activity manifest in the
optical/infrared emission line spectrum.
I discuss recent multi-wavelength observations of two very low-mass
objects in the context described above: DENIS 1048-3956, a very
low-mass field star observed in the past to be a radio burst source,
and FU Tau A, the primary of an isolated young brown dwarf binary.