We review different theories on the formation of the large scale structure of the Universe. Special emphasis is put on the theory of inertial instability. We show that, for a large class of initial spectra, the resulting two point correlation functions are similar. We also discuss the adhesion theory which uses the Burgers equation, Navier-Stokes equation or coagulation process. We review the Zeldovich theory of gravitational instability and discuss the internal structure of pancakes. Finally, we discuss the role of the velocity potential in determining the global characteristics of large scale structure (distribution of caustics, scale of voids, etc.). In the last section, we list the main unsolved problems and the main successes of the theory of formation of large scale structure.