Supernova explosions are a basic process that define the structure of the ISM. The interaction between supernova remnants (SNR) and the interstellar medium (ISM) is complex and symbiotic. Unlike other well-known SNR's, e.g., Cas A, Tycho, or Kepler, the structure of the Cygnus Loop is dominated by its interaction with the ISM. The Cygnus Loop is the prime candidate for a case study of a SNR interacting with a multiphase medium; it is close and has a low foreground column ensuring unrivaled signal-to-noise and spatial resolution. From an observational perspective the Cygnus Loop is unique in terms of X-ray brightness, shock front resolution ( ), and interaction with a range of ISM environments. The large diameter of the Cygnus Loop ( ) assures that it is interacting with different components of the ISM: from molecular gas on the NW edge (Scoville et al. 1977), to diffuse atomic gas along the NE and eastern limbs (DeNoyer 1975; Hester et al. 1994), and low density, hot, ionized gas to the south (Ku et al. 1984).
High resolution X-ray observations are crucial for studying SNR gas dynamics because: 1) shock codes predict fully developed turbulence when the blastwave collides with a cloud (Klein 1996); 2) the length scale, , for cooling and recombination is small: , where is the preshock number density, and is the shock velocity in units of (McKee 1987); 3) clouds sizes and substructure range from scales of tens of pc and downward. Consequently, the improved ROSAT-HRI sensitivity and spatial resolution compared to Einstein make it invaluable for exploring cloud-blastwave interactions. The ROSAT-HRI combines high 0.1-2 keV sensitivity ( better than the Einstein HRI [Zombeck et al. 1990]) and high resolution ( on axis [ROSAT Status Report #85]).