Much interest has developed in the use of sticky yellow boards to control the greenhouse whitefly. Such yellow boards are now produced for sale by several commercial companies. This paper summarizes experiments that assess the importance of various factors affecting trap efficacy, such as colour, position and quality of adhesive. Because the experiments were conducted under natural greenhouse conditions, growers will find the recommendations directly applicable. The most important variable is colour. Bright yellow was most attractive, having a reflectance spectrum with little or no reflectance in the blue-violet, a sharp rise beginning at 500 nm, reaching 30% at 540 nm and 90% at 570 nm, and continuing this level of reflectance into the far red. Boards should be positioned so that they hang down into the foliage, with their tops roughly even with the canopy. The adhesive should be chosen with care, since powdery wings of whiteflies allow them to escape from many adhesives. Several polybutene-type adhesives were suitable.