Inflation and Birth of Cosmological Perturbations

We review recent developments in the theory of inflation and cosmological perturbations produced from inflation. After a brief introduction of the standard, single-field slow-roll inflation, and the curvature and tensor perturbations produced from it, we discuss possible sources of nonlinear, non-Gaussian perturbations in other models of inflation. Then we describe the so-called $\delta N$ formalism, which is a powerful tool for evaluating nonlinear curvature perturbations on super Hubble scales.