The suite of open source software tools known as “oTree” makes it simple to conduct behavioral experiments online or in laboratories. (The word “Tree” in the name refers to decision trees, and the prefix “o” stands for “open.”) Without the need for sophisticated programming, researchers can easily build and run games on oTree that test all kinds of hypotheses about human decision-making. This grant funds a project by Toulouse economics professor Daniel Chen to expand oTree’s capabilities. Planned improvements include handling large-scale experiments, supporting continuous-time games, integrating oTree with other open source tools, improving documentation, diversifying its users and funders, and enhancing its long-term sustainability.