It is pointed out that the performance of speckle imaging or optical interferometer systems increases with (r sub 0/D) exp n, where r sub 0 is the atmospheric coherence length, D is the aperture size, and n is between 2 and 4. It has been determined that, since r sub 0 is about 10 cm at visible wavelengths and D may be several meters, selecting a site with a large r sub 0 becomes critical for 30-100-m baseline systems. A unique problem for such optical systems is the need for a relatively large, flat, approximately 100-m site; however, this is inconsistent with the atmospheric dynamics that produce optical sites. Albuquerque and Chilao Flats results indicate that katabatic flows produce r sub 0 values of 30-50 mm; on the other hand, large mountain tops tend to have large 50-200 m inner layers, making r sub 0 extremely sensitive to the surface heat flux and wind speed. It is concluded that few locations can achieve this; those along the California Pacific Coast and Mauna Kea are two such regions.