This chapter briefly reviews the development of traffic flow theory from the perspectives of macroscopic,mesoscopic,microscopic,and stochastic approaches.Some representative models and methods are summarized.Particularly,characteristics of the stochastic traffic flow modeling will be emphasized and analyzed in detail.Since headway/spacing are the fundamental parameters highly correlated with traffic flow rate,density and speed,then probabilistic modeling of headway/spacing distributions will be categorized such as univariate distribution,compositional distribution,mixed distribution,and random matrix theory based approach.The joint distribution of headway/spacing/velocity can be regarded as a basis for stochastic modeling of traffic state evolutions.The historical development of traffic flow theory forms a solid theoretical background for the whole book.