A pie crust consists of the outer shell of a pie. Some varieties are made of pastry dough, while "crumb crusts" are made of cookie or graham cracker crumbs held together with fat. Varieties made with pastry dough are typically mild in flavor and can either be flaky or somewhat crisp and crunchy. Crumb crusts are generally crumblier, and they will have the flavor of whatever cookie is used to make them.
The underside of a pie is always covered. The top may be uncovered (open-topped), fully covered by a second crust, or covered by a lattice top crust, which covers some areas while leaving the spaces between the lattices exposed. A non-crust method to top pies can be achieved through a crumb or streusel topping, which is a rich topping made of loosely blended ingredients and which, unlike pastry, has little structural integrity.