Building on a talk delivered with Anita Wager at NCTM, I will use the notion of a zeroth law in physics -- a principle that comes before all others -- to argue that joy is foundational to both the content and practice standards in mathematics. Attending to joy provides a powerful signpost for making decisions about both research and teaching. I will unpack the concept of joy by examining a variety of classroom interactions, demonstrating that joy comes not only from engaging in mathematics in conjunction with other pleasurable activities, such as talk and play, but also from the work of mathematics itself.