In a mixing bowl, beat the eggs, then stir in the pumpkin, sweetened condensed milk, zest, and pumpkin pie spice.
Pour the filling into the pie crust.
Bake the pie for 15 minutes.
Turn the oven temperature down to 350 F and make a loose foil tent over the pie. Continue to bake the pie for 45 to 55 minutes, or until a toothpick inserted into the center comes out clean.
Let the pie cool for at least 2 hours before slicing.
Serve the pie with whipped cream and a sprinkling of additional spices, if desired.