Pie crust is the shell which forms the outside of a pie. Old-style pie crusts are made of pastry. They are usually bland, flaky, and somewhat greasy. Many modern pies use crumb crusts, consisting of cookies crumbs or graham cracker crumbs held together with fat. Crumb crusts are generally sweeter and more flavorful than pastry ones.
The underside of a pie is always covered. The top may be uncovered (open-topped), fully covered by a crust, or covered by a lattice top crust, which covers some areas while leaving the spaces between the lattices exposed. A non-crust pie topping is a crumb pie topping, which is a rich topping made of loosely blended ingredients, and which, unlike pastry, has little structural integrity.