Large factor models use a few latent factors to characterize the co-movement of economic variables in a high dimensional data set. High dimensionality brings challenge as well as new insight into the advancement of econometric theory. Due to its ability to effectively summarize information in large data sets, factor models have been increasingly used in economics and finance. The factors, being estimated from the high dimensional data, can help to improve forecast, provide efficient instruments, control for nonlinear unobserved heterogeneity, and capture cross-sectional dependence, etc. This article reviews the theory on estimation and statistical inference of large factor models. It also discusses important applications and highlights future directions.