OpenSesame is a free, open-source, cross-platform graphical experiment builder designed for the social sciences. It features a comprehensive and intuitive graphical user interface (GUI) and supports Python scripting for complex tasks. Additional functionalities, such as support for eyetrackers, input devices, and video playback, are available through plug-ins. OpenSesame can be used in combination with existing software for creating experiments. The article provides a non-technical description of OpenSesame's functionality, compares it with existing software, discusses timing considerations, and presents the results of a benchmark experiment to evaluate its timing precision. OpenSesame is available for download from http://www.cogsci.nl/opensesame, with documentation, tutorials, example experiments, and plug-ins available online. The article highlights OpenSesame's unique features, including its fully graphical environment, extensibility through plug-ins, support for Python scripting, interoperability with existing software, and cross-platform compatibility.OpenSesame is a free, open-source, cross-platform graphical experiment builder designed for the social sciences. It features a comprehensive and intuitive graphical user interface (GUI) and supports Python scripting for complex tasks. Additional functionalities, such as support for eyetrackers, input devices, and video playback, are available through plug-ins. OpenSesame can be used in combination with existing software for creating experiments. The article provides a non-technical description of OpenSesame's functionality, compares it with existing software, discusses timing considerations, and presents the results of a benchmark experiment to evaluate its timing precision. OpenSesame is available for download from http://www.cogsci.nl/opensesame, with documentation, tutorials, example experiments, and plug-ins available online. The article highlights OpenSesame's unique features, including its fully graphical environment, extensibility through plug-ins, support for Python scripting, interoperability with existing software, and cross-platform compatibility.