z-Tree is a widely used software package for developing and carrying out economic experiments. The language used to define the experiments is simple and compact, meaning that experiments can be developed quickly, and programming experience is not necessary, though useful. z-Tree is flexible both with respect to the logic of interaction and the visual representation, allowing the simple programming of normal form games, extensive form games, double auctions, or clock auctions, for example. We began the development of the software in 1998, and have continually added new features.