For protected cultivation, where mechanical ventilation is not usually employed for energy and maintenance reasons, natural ventilation is the most inexpensive way to regulate greenhouse internal microclimate.Natural ventilation systems, however, offer a limited control over the air flow through the greenhouse. As a result, there are difficulties in controlling the indoor temperature, the relative humidity and the carbon dioxide concentration. Therefore a deeper analysis of the mechanisms of natural ventilation is necessary if we are to understand the phenomenon and the role played by wind and thermal buoyancy,which are its basic driving forces.