Rinse cranberries and remove any soft or damaged berries. Set aside.
In a medium saucepan, combine sugar and water. Bring to a boil over medium-high heat, stirring until sugar dissolves completely.
Add cranberries to the boiling sugar water. Return to a boil, then reduce heat to medium.
Cook uncovered for 10-12 minutes, stirring occasionally, until cranberries burst and the mixture thickens. You'll hear them pop!
Remove from heat and stir in orange zest if using. The sauce will continue to thicken as it cools.
Transfer to a serving bowl and let cool to room temperature. Refrigerate until ready to serve. Can be made up to 4 days ahead.