Modern science and medicine, and increasing prosperity, have brought many benefits to our society, but not without costs. One cost is that some of the technological innovations and new activities bring with them different and perhaps greater risks that threaten our lives. Any problem involving life- threatening risks is complex, so it is difficult to think through all the implications of the alternatives proposed to address it. This paper presents a number of 'facts' to guide constructive thinking about decisions concerning life-threatening risks. The intent is to help us appraise alternatives, design public policy and laws, and communicate about specific risk problems.