Until the middle of the 20th century, a reader would have stumbled against the phrase “Human Computation” because, at that time, computers were humans carrying out calculations, not machines. Back then, a special issue on “Machine Computation” would have aroused much interest. Nowadays, things are the other way around: A reader is likely to stumble against the phrase “Human Computation” because it is, nowadays, common knowledge that machines outperform human beings in a wide range of tasks; one might even wonder how humans could contribute to computations in a useful manner! Indeed, many human skills are far away from being fully taken over by machines. For example, reading comprehension, image recognition, or finding heuristic solutions for complex computational tasks like the traveling salesman problem still is beyond the capabilities of machines. Such tasks still are within humans’ reserved domain. It therefore seems natural that combining both the skills of humans and of machines can result in a higher problem solving competence both in quantity and quality. This insight paves the way for “Human Computation” as we know it today. This special issue introduces “Human Computation” through presentations of current research projects. The first article, “Mobile Learning in Environmental CitizenScience:An initial survey of current practice inGermany”, reports on a survey of mobile learning among environmental citizen science projects. This first article offers a good overview of why projects of very different types rely on “Citizen Science”, a form of “Human Computation”. The second article, “Design and Implementation of a Platform for the Citizen Science Project Migraine Radar”, describes a Citizen Science platform based on a software