How far is ‘infinity’?

Our present knowledge of the distribution of matter in the local universe is reviewed. Appropriate boundaries isolating astrophysical systems are sought on the length scales of the solar system, the galaxy and the local group of galaxies. The influence of diffuse matter is compared to that of nearby objects using the geodesic deviation equation. The question of assigning realistic wave zones to some canonical sources of gravitational radiation is briefly reviewed. Taking our local environment as typical, it is found that compact systems of a size similar to that of the solar system can normally be considered isolated. Compact galactic nuclei with low matter flux can probably also be considered isolated.