Hi Barbara, It is usually not necessary to cook fresh cherry pie filling before adding to the pie shell and baking the pie. But some bakers do prefer to give the cherries a head start by cooking them first, so they break down more in the filling. But you also bring up the eternal question – how do you thicken your pie? For fresh cherry pie, my answer is neither cornstarch nor flour, but tapioca. I prefer tapioca because cornstarch and flour have a tendency to make the pie juices cloudy and I really like the bright, ruby-like color of cherry filling that you get with tapioca (or arrowroot, for that matter). The trade-off is that the filling may not set as firmly when the pie is cooled as if you used flour. But I don’t mind a slightly runny filling. FYI, for 4 to 5 cups of pitted fresh cherries, you’ll need 3 to 4 tablespoons tapioca. And be sure to let the filling ingredients (including cherries, sugar, tapioca, and any liquid) sit for about 15 minutes to let the tapioca do its job of thickening before adding to the pie shell.