Half of the world's population has internet access. In principle, researchers are no longer limited to subjects they can recruit into the laboratory. Any study that can be run on a computer or mobile device can be run with nearly any demographic anywhere in the world, and in large numbers. This has allowed scientists to effectively run hundreds of experiments at once. Despite their transformative power, such studies remain rare for practical reasons: the need for sophisticated software, the difficulty of recruiting so many subjects, and a lack of research paradigms that make effective use of their large amounts of data, due to such realities as that they require sophisticated software in order to run effectively. We present Pushkin: an open-source platform for designing and conducting massive experiments over the internet. Pushkin allows for a wide range of behavioral paradigms, through integration with the intuitive and flexible jsPsych experiment engine. It also addresses the basic technical challenges associated with massive, worldwide studies, including auto-scaling, extensibility, machine-assisted experimental design, multisession studies, and data security. Keywords Online studies. Robust and reliable research. Massive online experiments. Citizen science Although some questions psychologists care about involve comparing only two conditions to each other, most require teasing apart the contributions of many intertwined variables. In the past, this has required hundreds, if not thousands, of studies across numerous laboratories, each targeting a specific variable, population, or stimulus set. In principle, we can now do this work many orders of magnitude more quickly. Given that half the world's population has internet access (ITU Telecommunication Development Sector, 2017), any study that can be run on a computer or mobile device can be run with nearly any demographic anywhere in the world, and in large numbers. This includes not just surveys, but studies involving grammatical judgments, reaction times, decisionmaking, economics games, eyetracking, priming, sentence completion, skill acquisition, and others-which is to say, most human behavioral experiments (Birnbaum, 2004;