Concern has been expressed at successive Review Conferences of the Biological and Toxin Weapons Convention (BTWC) regarding the possible misuse of new biological knowledge; this article reviews some of these developments. Genetic manipulation, and genomics in particular, would modify existing pathogens and render previously harmless organisms pathogenic. Viruses could be modified as vectors to alter their pathogenicity in animals and man or act as carriers for genes or toxins. Plant pathogens, particularly fungi, could be modified as biological warfare agents against crops. An effective verification protocol for the BTWC is an essential part of the web of deterrence against these developments.