  The observation of gaseous giant planets is of high scientific interest.
Although they have been the targets of several spacecraft missions, there still
remains a need for continuous ground-based observations. As their atmospheres
present fast dynamic environments on various time scales, the availability of
time at professional telescopes is neither uniform nor of sufficient duration
to assess temporal changes. However, numerous amateurs with small telescopes
(of 15-40 cm) and modern hardware and software equipment can monitor these
changes daily (within the 360-900nm range). Amateurs are able to trace the
structure and the evolution of atmospheric features, such as major
planetary-scale disturbances, vortices, and storms. Their observations provide
a continuous record and it is not uncommon to trigger professional observations
in cases of important events, such as sudden onset of global changes, storms
and celestial impacts. For example, the continuous amateur monitoring has led
to the discovery of fireballs in Jupiter's atmosphere, providing information
not only on Jupiter's gravitational influence but also on the properties and
populations of the impactors. Photometric monitoring of stellar occultations by
the planets can reveal spatial/temporal variability in their atmospheric
structure. Therefore, co-ordination and communication between professionals and
amateurs is important. We present examples of such collaborations that: (i)
engage systematic multi-wavelength observations and databases, (ii) examine the
variability of cloud features over timescales from days to decades, (iii)
provide, by ground-based professional and amateur observations, the necessary
spatial and temporal resolution of features that will be studied by the
interplanetary mission Juno, (iv) investigate video observations of Jupiter to
identify impacts of small objects, (v) carry out stellar-occultation campaigns.
