Complex cycling workflows are fundamental to numerical weather prediction (NWP) and related environmental forecasting systems. Large numbers of jobs are executed at regular intervals to process new data and generate new forecasts. Dependence between these forecast cycles creates a single never-ending workflow, but NWP workflow schedulers have traditionally ignored this-at the cost of efficiency when running “off the clock”-by enforcing a simpler nonoverlapping sequence of single-cycle workflows. Cylc (“Silk”)1 -3 is designed to manage infinite cycling workflows efficiently even after delays in real-time operation, or in historical runs, when cycles can typically interleave for much-increased throughput. Cylc is not actually specialized to environmental forecasting, however, and cycling workflows may also be useful in other contexts. In this paper, we describe the origins and major features of Cylc, future plans for the project, and our experience of Open Source development and community engagement.@en