ariadne was founded to catalyze manual image annotation, in particular neuron reconstruction efforts in the neuroscience community, by outsourcing image annotation tasks to full-time professionals. For most image annotation tasks in biology, 20–40 hours of training are necessary to become familiar with the task, software, and data. Large-scale image annotation pipelines that rely on students or volunteers usually suffer from a high turnover of workers, low individual throughput, and low or mediocre annotation quality, due to workers’ lack of long-term experience. Consequently, these pipelines involve a significant training and management overhead. ariadne minimizes overhead costs and maximizes annotation quality and throughput by maintaining a group of highly trained full-time annotators.