Drawing on the work of the National Academies, this booklet introduces some of the issues at the intersection of science and security. The life sciences offer tremendous promise for meeting many 21st century challenges. But with opportunities come responsibilities. An important aspect of scientists' responsibility to society is captured in the concept of biosecurity-- the challenge to move the life sciences forward for legitimate purposes while reducing the risks that some materials, knowledge, tools, and technologies could also be used to do harm. This booklet was developed to serve as an educational resource for students and the scientific community, to illuminate the importance of biosecurity, and to explore how scientists, organizations, and governments at many levels can work together to minimize the threat.