Human computation is a computing approach that lets humans perform tasks for which there's still no satisfactory solution, even when today's most sophisticated computing infrastructures are used. One stream of human computation is âvolunteer thinking,â' systems that gather volunteers willing to contribute by executing human computation tasks in citizen-science systems. The authors report the findings of a volunteer engagement characterization study of two astronomy projects: Galaxy Zoo and the Milky Way Project. Approximately 10 million tasks executed by 100,000 volunteers over a near-two-year time period form the basis of the study. Their results show a diversity of engagement patterns in terms of frequency, daily productivity, typical session duration, and the amount of time devoted to the project. The authors discuss the implications of such patterns and offer suggestions to researchers who are seeking to take advantage of this computing approach.