Midpen preserves cover the spectrum from redwood forests and chaparral-covered hillsides to sensitive wetlands along the San Francisco Bay. Ranging from 55 to over 18,000 acres, they are open to the public free of charge, 365 days a year, from dawn to one-half hour after sunset.

Open space preserves are generally kept in a natural condition in order to best protect the environment and wildlife habitat, and are developed with only the amenities needed to provide low-intensity recreation. Improvements may include paved or gravel parking areas, vault restrooms, signed trails for hiking, bicycling, and equestrian and dog use, and an occasional picnic table.