In a small saucepan over medium heat, bring 1 cup water to a boil. Stir in granulated sugar until dissolved. Let cool.
In a large pitcher, stir lemon juice, cooled simple syrup, and 8 cups cold water. Serve over ice. Garnish with lemon slices.
Make Ahead: Simple syrup can be made 1 week ahead. Store in an airtight container and refrigerate.