The provision of an airflow to assist in the distribution of pesticides within crop canopies has led to a number of sprayer designs with different fan systems and configurations of a range of nozzle types. Generally, the interest in the use of air-assisted sprayers has increased since Potts (1958) recognised the ability to reduce spray volumes significantly by using an airflow to project droplets into a crop canopy. Traditionally used for treating tree crops, air-assisted sprayers have now been adopted in many other types of crop. Downwardly directed air assists droplet penetration of arable field crop canopies and in reducing downwind drift (Figure 8.1).However, registration authorities are concerned that when spraying orchards, small droplets can drift above the tree canopy (Gil et al., 2007(Gil et al., , 2008 and outside orchards so buffer zones are wider for this type of application. Reyes et al. (2012) have developed a data acquisition system to assess the quality of spraying and verify whether the weather conditions were appropriate when a spray was applied.Various terms have been used in association with air-assisted spraying. These include 'concentrate', 'mistblower' and 'air-carrier' spraying. Mistblowers are sprayers that produce droplets in the 50-100 μm size range as these droplets are most effectively conveyed within an airstream. Potts (1958) found that in a particular airstream, droplets of 60-80 μm diameter were carried 46 m, while the larger 200-400 μm droplets travelled only 6-12 m. Larger droplets will be influenced more by gravity, while the smallest droplets are less likely to impact on foliage and other targets as they remain within the airstream. This is particularly important when projecting spray upwards into a tree canopy as fall-out due to gravity can result in considerable wastage of pesticide on the ground as well as increasing risk of operator exposure to the pesticide. However, with greater concern about spray drift out of orchards, coarser sprays are increasingly selected. The distance that large droplets are transported depends very much on the strength of the air assistance and the initial direction of trajectory of the droplets. As the risk of spray drift from orchards is considered to be greater than from arable crops, the unsprayed buffer zone (UBZ) in the UK is at least 18 m, although when using a tunnel sprayer, this is reduced to 5 m.