The effectiveness of large telescopes is limited by the terrestrial atmosphere rather than by their optical or mechanical performance. Due to its refractive, absorbing and scattering properties, the atmosphere is one of the important elements that determines the limiting magnitude, resolving power and the wavelength range within which observations are possible. These effects are present even on the clearest nights. In addition, general meteorological conditions such as cloudiness, wind, high humidity, or extremely low temperatures further impair or prevent the operation of astronomical equipment. These effects can be avoided by placing the equipment above most or all of the atmosphere (balloon-borne telescopes, satellite observatory, lunar observatory). For the foreseeable future the cost of placing and operating a telescope outside the atmosphere will be such that it will be restricted to the investigation of problems that cannot be solved with observations made only from the ground. It seems possible, however, to choose sites for terrestrial observatories where the atmospheric effects are minimized. Just where the optimum conditions can be found is not yet known, but it is highly probable that none of the existing observatories is located on the World's best site. In fact, it is quite possible that conditions may be found on hitherto unknown sites that could bring a considerable advance in astronomical research before going to space equipment. Whether or not it is practicable to erect and operate an astronomical observatory on the optimum site (or sites) will depend largely on location, because in addition to excellent astronomical observing conditions the site has to be accessible and capable of development for satisfactory working conditions.