Understanding the atomic-to-molecular (HI-to-H2) transition is of fundamental importance for the theory of star-formation and the evolution of galaxies. I will discuss our recent theory of the HI-to-H2 transition in the turbulent interstellar medium. The turbulence, when supersonic, produces strong density fluctuations in the gas, which strongly affects the HI/H2 structure by varying the density-dependent H2 formation/destruction balance. I will show how 21 cm observations of the atomic gas may be used to constrain important turbulent parameters - the sonic Mach number and the turbulence driving scale.