This collection of articles, which constituted a special issue of the 'Journal of Artificial Intelligence', presents the most recent work on qualitative reasoning about the real (physical) world. A common theme of all the contributions is explaining how physical systems work-from heat flow to transistors to digital computation. The explanations are so detailed and exact that they can be used by computer programs to reason about physical work in the same kinds of ways that people do. This rapidly developing area of cognitive science, variously called Qualitative or Naive Physics, is of direct psychological interest and has strong connections to theories of linguistic semantics. But there are also immediate technological applications, with much of this work funded by industry in the expectation of building computer systems which can communicate sensibly with people about physical mechanisms.