OpenSesame is a free, open-source, cross-platform graphical experiment builder for the social sciences. It features an intuitive graphical user interface (GUI) and supports Python scripting for complex tasks. Additional functionality, such as support for eyetrackers, input devices, and video playback, is available through plug-ins. OpenSesame can be used in combination with existing software for creating experiments. OpenSesame is unique in that it is free, cross-platform, and offers the most intuitive and comprehensive GUI currently available. It supports Python scripting for complex tasks. A wide range of experiments can be created, including psychophysical experiments, speeded response time tasks, eye-tracking studies, and questionnaires. The article describes the functionality of OpenSesame, its comparison with existing software, and timing considerations. It also describes the results of a benchmark experiment. OpenSesame is freely available for download from http://www.cogsci.nl/opensesame. Documentation, a step-by-step tutorial, example experiments, and plug-ins can be found in the documentation area at http://osdoc.cogsci.nl/. The version of OpenSesame reviewed in the present article is 0.24. At the time of writing, OpenSesame has been downloaded more than 10,000 times (nonunique downloads from cogsci.nl and via bit-torrent), and there is an active support forum. OpenSesame does not impose strict system requirements. It has been extensively tested on Windows XP/Vista/7 and Ubuntu/Debian Linux. At the time of writing, packages for Mac OS are also available but have been labeled “experimental” pending further testing. On other platforms, users will need to manually install the software on which OpenSesame depends and run OpenSesame from the source code. Instructions for running OpenSesame from source are provided online. The processing power required to run OpenSesame depends strongly on the type of experiment. For a typical experiment, consisting of a sequence of static stimulus displays followed by response collection, the requirements are very modest, and any relatively modern computer system will suffice. When using complex stimuli, such as high-definition video, the user will need to evaluate for him- or herself whether the computer system is up to the task. The GUI of OpenSesame is described, including the structure of the experiment as a tree structure in the overview area. Items can be added to the experiment by dragging them from the item toolbar onto the overview area. Items can be edited by selecting them in the overview area, after which the appropriate controls appear in the tab area. Variables and conditional ("if") statements are used to add flexibility to the GUI. Variables can be built in, set by items, or specified by the user in loop items. These variables can be used throughout the GUI by entering [variable_name] in places where you would normally encounter a static value. ConditionalOpenSesame is a free, open-source, cross-platform graphical experiment builder for the social sciences. It features an intuitive graphical user interface (GUI) and supports Python scripting for complex tasks. Additional functionality, such as support for eyetrackers, input devices, and video playback, is available through plug-ins. OpenSesame can be used in combination with existing software for creating experiments. OpenSesame is unique in that it is free, cross-platform, and offers the most intuitive and comprehensive GUI currently available. It supports Python scripting for complex tasks. A wide range of experiments can be created, including psychophysical experiments, speeded response time tasks, eye-tracking studies, and questionnaires. The article describes the functionality of OpenSesame, its comparison with existing software, and timing considerations. It also describes the results of a benchmark experiment. OpenSesame is freely available for download from http://www.cogsci.nl/opensesame. Documentation, a step-by-step tutorial, example experiments, and plug-ins can be found in the documentation area at http://osdoc.cogsci.nl/. The version of OpenSesame reviewed in the present article is 0.24. At the time of writing, OpenSesame has been downloaded more than 10,000 times (nonunique downloads from cogsci.nl and via bit-torrent), and there is an active support forum. OpenSesame does not impose strict system requirements. It has been extensively tested on Windows XP/Vista/7 and Ubuntu/Debian Linux. At the time of writing, packages for Mac OS are also available but have been labeled “experimental” pending further testing. On other platforms, users will need to manually install the software on which OpenSesame depends and run OpenSesame from the source code. Instructions for running OpenSesame from source are provided online. The processing power required to run OpenSesame depends strongly on the type of experiment. For a typical experiment, consisting of a sequence of static stimulus displays followed by response collection, the requirements are very modest, and any relatively modern computer system will suffice. When using complex stimuli, such as high-definition video, the user will need to evaluate for him- or herself whether the computer system is up to the task. The GUI of OpenSesame is described, including the structure of the experiment as a tree structure in the overview area. Items can be added to the experiment by dragging them from the item toolbar onto the overview area. Items can be edited by selecting them in the overview area, after which the appropriate controls appear in the tab area. Variables and conditional ("if") statements are used to add flexibility to the GUI. Variables can be built in, set by items, or specified by the user in loop items. These variables can be used throughout the GUI by entering [variable_name] in places where you would normally encounter a static value. Conditional