The international community has laid down clear red lines about the use of biology to enhance national armaments. Advances in bioscience and biomedicine are, however, significantly eroding technological barriers to acquiring and using biological weapons. This article describes recent scientific trends and analyses their security implications. Three emerging fields of research that have particularly high potential for misuse are considered in more detail: potentially pandemic pathogens, synthetic biology and neurobiology. It is argued that continued efforts are required in multilateral, national and scientific spheres to strengthen the red lines and to foster responsible science.