The spectral and temporal variability of the isolated X ray pulsar RX J0720.4 3125 The radio quiet thermally emitting X ray pulsar RX J0720.4 3125 belongs to a group of seven isolated neutron stars (often referred to as the ``Magnificent Seven ). They share similar properties such as pure thermal radiation (black body emission with broad absorption features in some cases) long pulse periods 3 12 s) and high magnetic field strengths (B~1e+13 Gauss) on their surfaces. These neutron stars are nearby (<500 pc) and up to a few million years old (i.e. relatively young and still hot) thus most of them have been identified with dim (25 28 mag) blue optical counterparts. RX J0720.4 3125 is unique among the ``Magnificent Seven as it shows long term variations in its spectral properties (temperature size of the emitting area and equivalent width of the broad absorption feature) and irregularities in the pulse period on time scales of years. The main two theories put forward to explain the behaviour of RX J0720.4 3125 are either free precession or a glitch a sudden event like a star quake or an impact of a massive object that released much energy in a short time. In this work most recent data from new observations with XMM Newton and Chandra as well as already existing archival data are analysed (altogether now covering a time span of almost twenty years).