Aphids are tempting subjects for investigation because they are often abundant, easily found, and unlikely to run away, and because of intriguing facts about their biology such as polyphenism, parthenogenesis, gall formation, and the production of a soldier morph. However, until now identification has presented a severe challenge to the beginner, because there are so many rather similar species, many of which have several different forms. The authors make this much needed Naturalists' Handbook accessible by confining themselves to aphids leaving on broad-leaved trees. This has reduced the problems of identification to manageable proportions and should encourage more field studies of this important and ubiquitous group of insects.