We present the first X-ray expansion measurement of the SNR Cas A .
We compared new ROSAT to archived Einstein HRI images with a separation of 17 years .
The remnant is seen to be expanding , on average , at a rate of 0.20 \pm 0.01 % yr ^ { -1 } , which is twice as fast as the expansion of the bright radio ring and two-thirds as fast as the ensemble of optical Fast Moving Knots .
This argues that we are observing different hydrodynamical structures in each band , although all of these components are located on the same patchy ring .
In addition , significant variations in expansion rate as a function of azimuth around the bright X-ray ring are found .
These findings are discussed in the context of two classes of models , involving either an ambient medium with a uniform or monotonically decreasing density away from the center , or a pre-existing circumstellar shell .