UEFA uses country coefficients, which are calculated in amazingly complicated and abstruse ways, to determine the number of Champions League places each country's football association gets. (The Europa League is now much simpler; pretty much everybody gets three teams in).
These rankings are used to determine the number of places not for the following year, but for two years out--thus, the rankings at the end of the 2015-16 season will determine the allocation for the 2017-18 CL. The meaningful dividing line is between rank 3 and 4, and between rank 6 and 7, because the top three associations get 4 teams in, the next three get 3 teams in, and then the next nine get two.
Spain, Germany and England are the top three at the moment, and while England could overtake Germany if the English sides continue to do well and the German ones don't, it's pretty clear those three will have four places.
Italy is far and away fourth and so will have three slots, but then it gets interesting. At the moment France (52.082) are fifth, Portugal (51.915) sixth, Russia (51.082) seventh. If Zenit defeat Benfica, and pick up a win in the next CL round, they could tighten things up even more. Portugal have Braga still in the EL, though, which could help. France, of course, have only PSG left, and they carry a narrow 2-1 lead to Stamford Bridge to face Chelsea.
And since the coefficients are calculated by adding together the last five years, and when the 2011-12 season drops off Russia and France will be virtually tied anyway, expect Russia to pick up another CL berth in 2018-19 at the expense of either France or Portugal.