With a focus on gameplay first, Blizzard Entertainment is known for developing premium games like World of Warcraft, Starcraft, Diablo, Hearthstone, Heroes of the Storm and Overwatch. Tens of millions of players login daily and interact with a variety of game features generating massive amounts of rich and diverse data streams. In this talk, I will provide a general overview of data science challenges at Blizzard and discuss two challenges in the area of game design. Specifically, I will discuss challenges and solutions for matchmaking in competitive games; and discuss how gameplay and player segmentation can be used to inform game balance.