Harbor porpoise (Phocoena phocoena) are found in coastal waters of the temperate northern hemisphere, where they are subjected to a variety of anthropogenic impacts including pollution, noise, and fishery interactions. Marine renewable energy projects currently in development in California have the potential to disturb, displace, and damage the hearing of harbor porpoises. Effective monitoring strategies are necessary to establish baseline data prior to the installation of marine renewable energy structures, to determine the impacts of construction activities, and to assess long-term impacts from marine renewable energy site operation. Along the U.S. West Coast, harbor porpoise populations are monitored using line-transect aerial surveys, which are costly, weather-limited, and provide coarsely resolved data. However, passive acoustic monitoring is a promising new tool for more effective monitoring of this species along the U.S. West Coast. The authors proposed to establish a trial passive acoustic monitoring network for harbor porpoise and evaluate the feasibility of a passive acoustic approach to monitoring harbor porpoise during marine renewable energy site development in California. The authors sought to describe the relationship between acoustic and visual observations of harbor porpoise and to evaluate the optimal spatial and temporal sampling scales for an effective monitoring network. The authors found that a passive acoustic monitoring network for harbor porpoise is feasible to implement and is an improvement over traditional visual survey methods due to increased temporal sampling. Substantially more simultaneous visual and acoustic surveys would be required to characterize this relationship accurately. The data suggest that the spatial distribution of harbor porpoises shifts over periods of weeks and can be highly variable between years, and therefore collecting baseline data at the population level for several years prior to potential disturbance (as well as during and after) will be critical for accurately assessing the impacts of marine renewable energy installations on harbor porpoise populations.