  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 %/yr,
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.
