Since the COVID-19 pandemic emerged in March 2020, districts across the nation have faced the difficult task of reopening schools safely and keeping them open as community infection rates have risen and fallen. It is useful to learn from the efforts of counties and districts that have been able to reopen schools--and keep them open safely--using multifaceted mitigation strategies that greatly reduce the risk of in-school transmission. This brief describes reopening efforts in Marin County, which has reopened more than 85% of its schools since fall 2020, and documents the county planning process and mitigation strategies for personal protective equipment, ventilation, physical distancing, cohorting, symptom screening, and testing and tracing. It is part of an effort at the Learning Policy Institute to disseminate key public health research and reopening strategies to educators and policymakers.