The use of screening to reduce insect entry into greenhouses is becoming a common practice, particularly in commercial production facilities in warm climates. Reducing the use of pesticides has the potential to save money and to reduce chances for environmental pollution outside the greenhouse facility. In an era of increasing environmental awareness on the part of the public and its insistence for more stringent regulatory control, non-chemical means of crop protection are increasing in importance. The prevention of movement of insects into or out of quarantine greenhouse facilities has long been a major consideration in their design. Recent advances in quarantine greenhouse design have provided some basis for improving the designs adopted for commercial greenhouse operations. The most common screening systems for mechanically ventilated greenhouses, with exhaust fans, involve the application of screening at the air inlet only. Attempts to screen passively or naturally ventilated greenhouses generally result in undesirably high air temperatures, in part because effective systems must screen both inlets and outlets. A positive pressure ventilation system with screening can offer several advantages over standard exhaust systems. Maintaining internal greenhouse pressures at high enough pressure that air velocities out through open doors, or other openings in the structure, exceed the flying speed of the insects of concern should be more effective in excluding insects than an exhaust system which tends to draw insects in through openings. With proper design, insect exclusion can be achieved with only one application of screening at the air inlet and small openings in the greenhouse glazing should not provide easy entry for insects. This concept is particularly attractive for use in warm seasons and tropical or subtropical areas where modest airflow rates can be used to maintain positive internal greenhouse pressures throughout the day.