In recent years, scientists have made huge gains in their understanding of how genes can be altered and transferred from one organism to another - but that knowledge has been acquired amidst controversy and concern, as the front cover illustrates. The deep ethical concerns that have resulted from the emergence of genetic manipulation are explored in this topic. We begin with an examination of the basic structure and function of genes. A number of pioneering examples and techniques are explored, helping to explain why our present-day view of genetic manipulation can combine feelings of optimism and unease. Examples are drawn from both plants (notably GM crops) and animals (including Dolly the sheep), with a special emphasis on the implications of promising medical techniques such as gene therapy. Our hope is that by exploring the science 'behind the headlines', and its interactions with the equally complex social factors, we will acquire a clearer idea of both what is possible and what may be desirable.