The idea of cosmic inflation has played a central role in the development of cosmology over the last 20 years. Inflation offers an explanation for many features of the Universe that used to seem beyond the reach of scientific explanation, in particular its spatial flatness and homogeneity, and makes specific predictions for the seeds that formed galaxies and other structure. (For reviews see for example [Albrecht(1999), Liddle and Lyth(2000), Linde(1990)])
The emergence of the Cosmic Inflation idea has created many exciting opportunities. On the astronomical side, it has given us a "standard model" with specific values for the density of the Universe, tot, and the spectrum of deviations from perfect homogeneity, predictions that can now be confronted with astronomical data. The prospect of testing these predictions has been crucial to making the case for the host of new observational campaigns (such as the Sloan Digital Sky Survey and the MAP and PLANCK satellites) which are set to produce a flood of new data.
There are also key unresolved questions within the inflationary picture. Progress on these questions falls squarely in the domain of high energy physics and is likely to lead to additional opportunities for observational tests. One significant question is whether there is any real competition for cosmic inflation: are there alternative dynamical processes that could, like inflation, set up the Universe with the features we observe? If so, how can we tell which of the alternatives Nature might have actually chosen? Recent attempts to address these questions have stimulated considerable interest and have posed problems for fundamental physics that are exciting in their own right.