The Great Gray Owl (Strix nebulosa) is a difficult species to detect because of its remote breeding locations and secretive nature. We used autonomous recording units (ARUs) to monitor nest activity at 6 Great Gray Owl territories from June–July 2006 in the Sierra Nevada, California. We also used ARUs to survey for Great Gray Owl presence–absence status at 15 potential territories from March–April 2007. Each unit recorded 12 h/night (18:00–06:00). In 2006, we recorded juvenile begging calls at 3 sites. In 2007, we recorded adult Great Gray Owl vocalizations at 10 sites. We recorded audible calls during approximately 40% of the nights sampled at locations with owl detections. We concluded that a combination of ARUs and acoustic analysis provides an effective non-invasive method to detect and monitor Great Gray Owls as well as other secretive and nocturnal species.