The objective of using pesticides in greenhouses is to reduce or manage pest populations without endangering the lives of greenhouse workers or customers, and without harming plants. To achieve this goal, it is essential to maximize pesticide exposure to the targeted pests and minimize human pesticide exposure. The first part of this publication will address issues related to pesticide applications, whereas the second part addresses human safety issues. Pesticide Application The focus of a pesticide application is to deliver the pesticide to the target pest (i.e., insect, mite and pathogen). Most pesticides are sprayed onto plant foliage; however, some pesticides are available in granular formulations that can be incorporated into the growing medium or topdressed on the growing medium surface. Spray Application When a pesticide is sprayed, the material is dispersed into small droplets. The smaller the droplets, the greater the number of droplets per area of greenhouse, and the greater the likelihood of hitting the target pest (Table 1). Smaller droplets (10 to 50 microns) are more likely to contact flying insects (note: one micron equals 0.000004 inch). Droplets that are 30 to 50 microns are most likely to come into contact with insects on foliage, while the larger droplets (250 to 500 microns) are most useful at contacting insects in the growing medium. The type of spray equipment used determines the droplet particle size.