Abstract. Automated electronic data loggers revolutionized environmental monitoring by enabling reliable high-frequency measurements. However, the potential to monitor the complex environmental interactions involved in global change has not been fully realized due to the high cost and lack of modularity of commercially available data loggers. Responding to this need, we developed the ALog series of open-source data loggers, based on the popular and easy-to-program Arduino microcontroller platform. ALog data loggers are low cost, lightweight, and low power; they function between −30 °C and +60 °C, can be powered by readily available consumer-grade batteries and solar panels, and can store up to 32 GB of data locally. They are compatible with standard environmental sensors, and the ALog firmware library may be expanded to add additional sensor support. The end-product is a set of robust and field-tested scientific instrumentation that is the direct descendant of dozens of individuals' contributions to the growing open-source electronics movement.