Diesel soot, one of the major environmental pollutants, is the finer particle produced during the high temperature pyrolysis or combustion of diesel fuel. Advances in the understanding of soot formation in diesel combustion systems during the recent decades are surveyed in this paper. It is universally accepted that soot formation steps can be summarized as (1) formation of molecular precursors of soot, (2) nucleation or inception of particles from heavy polycyclic aromatic hydrocarbon molecules. (3) mass growth of particles by addition of gas phase molecules, (4) coagulation via reactive particle-particle collisions, (5) carbonization of particulate material, and, finally, (6) oxidation of polycyclic aromatic hydrocarbons and soot particles. Several mathematical models of diesel soot formation available in the related literature are offered in this review, which are Hiroyasu's model, Moss' model, Tesner's model, Lindstedt's model, and detailed soot formation models. Experimental and numerical studies currently play an important role in exploring the diesel soot formation mechanism. The success of such studies relies on the development of both diagnostic techniques to increase the quality of experiments and mathematical models for numerical simulations.