During the assembly of the vacuum vessel and the subsequent operation of JET hundreds of vacuum leaks were found and repaired. Information on leaks has been collected and evaluated since 1981 in an attempt to reveal the causes. In this evaluation a clear distinction was made between leaks occurring during the installation and during the plasma operation phase. Most installation leaks on flanges appear to be caused by incompatible seals, damage or contamination of the sealing surfaces. Leaks in welded connections were mostly due to impurities, incompatible welding materials or inaccurate setting of the welding parameters. The difficulty of access to flanged or welded connections proved to be responsible for the occurrence of leaks during assembly. Operational leaks are in general the result of design errors, problems with bellows, in particular bellows in water cooling systems, and mechanical forces on the equipment due to uncontrolled plasma behaviour (disruptions). Recommendations will be made on leak prevention for the radioactive tritium phase of JET and for future machines.