Alan H. Guth is the Victor F. Weisskopf Professor of Physics at MIT. Most of Professor Guth’s research has centered on the application of theoretical particle physics to the early universe: What can particle physics tell us about the history of the universe, and what can cosmology tell us about the fundamental laws of nature? In 1981 he proposed that many features of our universe, including how it came to be so uniform and why it began so close to the critical density, can be explained by a new cosmological model which he called inflation. Inflation is a modification of the conventional big bang theory, proposing that the expansion of the universe was propelled by a repulsive gravitational force generation by an exotic form of matter. Although Guth’s initial proposal was flawed (as he pointed out in his original paper), the flaw was soon overcome by the invention of “new inflation,” by Andrei Linde in the Soviet Union and independently by Andreas Albrecht and Paul Steinhardt in the US. After more than 20 years of development and scrutiny the evidence for the inflationary universe model now looks better than ever.