Note: Key lime juice is usually available near the Real Lemon juice in grocery stores. Key lime pie is actually yellow in color. You may add a few drops of green food coloring if you like or simply keep it natural.
Stir together 8 ounces whipped topping (Cool Whip), sweetened condensed milk, and key lime juice. Pour into pie shell. Garnish with a bit more whiped topping or a few lime slices. Refrigerate for at least 2 hours before serving.