Rinse one medium cucumber under cold water. Use a sharp knife or a mandoline to slice the cucumber thinly lengthwise.
Place the cucumber ribbons in a large pitcher. Pour in 8 cups of cold water and stir gently to combine. Cover and refrigerate for at least 1 hour to let the flavors meld.
When ready to serve, add ice to glasses and pour the infused water over. Garnish with lemon slices, if desired.