  Over the past few years monitoring programs have shown ultra cool dwarfs
(UCDs) to be photometrically variable. Of the 60 sources monitored in the field
and some 120 monitored in clusters, about 40% show variability in both cases.
For mid to late M dwarfs in young (<100 Myr) clusters, this variability is
generally periodic with amplitudes of up to a few tenths of a magnitude and
periods of between a few hours and several days. For older field dwarfs
(covering late M, L and T types) this variability is often nonperiodic with
smaller amplitudes (up to 0.1 mag in I) and timescales of order hours. The
former may be attributed to the rotational modulation of magnetically-induced
photospheric spots, as seen in higher mass T Tauri stars. The nonperiodic
variability, on the other hand, may be caused by a rapid evolution of surface
features (which 'mask' the otherwise observable rotational modulation). This
could be related to the formation and dissipation of inhomogeneities in dust
clouds in the photospheres of UCDs.
