A hydraulically controlled packer for an oil or gas well is disclosed that i s capable of being hydraulically unset. One embodiment of the invention is capable of being hydraulically unset prior to locking thus allowing fine tuning of packer location in relation to oil-bearing strata. Other embodimen ts of the invention are capable of being hydraulically unlocked and unset for further use within the oil or gas well without being withdrawn to the surfac e for reassembly. The invention may be used in downhole well tools other than packers.
