  According to the paradigm of solar-type dynamo action brown dwarfs should not
exhibit magnetic activity as they are fully convective. Indeed, Halpha
observations of ultracool field dwarfs indicate a decline of activity setting
in near the substellar limit. X-ray emission traditionally serves as another
means to examine magnetic activity on stars. The substellar regime can now be
accessed with the new generation of X-ray instruments onboard XMM-Newton and
Chandra. We report on two recent XMM-Newton observations of brown dwarfs in the
Pleiades cluster and in the field aiming to constrain the age and effective
temperature dependence of X-ray emission from substellar objects.
