A review; time and length scales accessible to ab initio mol. dynamics simulations are necessarily limited. This is the price one pays for an "in principle" unbiased description of chem. reactivity. We address the problem of times scales focusing on the detn. of the reaction path and mechanism of chem. reactions using advanced sampling techniques. Further in the discussion about the calcn. of thermochem. consts. both, time and length scale problems are covered. Finally, new techniques for extending system sizes with d. functional theory are presented.