Abstract: A method for washing the interior surfaces of tanks with a washing head is provided with a nozzle (28) which ejects a liquid beam and which is rotatable about a first (11) and a second (34) axis whereby the nozzle is allowed to cover a two-dimensional solid angle. The washing head is controlled in such a manner that for an entire revolution about the first axis, the nozzle performs a small rotation about the second axis, and that over a number of revolutions about the first axis the nozzle follows a path which covers the entire a solid angle. The drive mechanism for rotating the washing head is arranged externally of the conduits carrying washing liquid in order to avoid loss of washing pressure and wear of the drive mechanism. According to the invention the movement of the nozzle along the defined path is monitored and the result of the monitoring is utilised to control the energy density of the beam in order that it is reduced when the beam is directed at the zones which do not require the maximum energy.