The idea that there was a period of accelerated expansion in the very early universe, inflation, was born more than 30 years ago. However, it was like a fairy tale until 1992 when a tiny temperature fluctuation was found in the Cosmic Microwave Background (CMB) radiation in the sky. The spectrum of this tiny fluctuation was found to be in very good agreement with the prediction of inflation. Since then observation has advanced considerably and today every detail of the temperature fluctuation is used to probe the very early stage of the universe, when its size was about 10^{-30} of the current size. Here cosmological perturbation theory, which has been also developed over the last 30 years, plays the central role. In this talk, I review these developments in the theory of inflation and its predictions, and touch on future prospects.