A by-election at this point in the Parliament usually tells us absolutely nothing, but rarely this one has been very revealing.
It's a stellar result for the Liberal Democrats, who have proved they are doughty local campaigners and there is a reasonable chance they can hold on to their seats at the next election. And who knows, maybe get a few more.
That leaves the Tories in a very bad position, because their chances of winning a majority depends on winning at least 20 or so Lib Dem seats. The evidence here shows that isn't very likely.
David Cameron hasn't always been particularly popular with his backbenchers and the big question from a lot of them will be: would we do better with a change?
Labour have got problems too, because it's reasonably clear that they're not an especially attractive alternative if people are disgruntled with the Coalition.