The working paper is divided into two main parts. The first part is a descriptive analysis of the illicit use of biological agents by criminals and terrorists. It draws on a series of case studies documented in the second part. The case studies describe every instance identifiable in open source materials in which a perpetrator used, acquired, or threatened to use a biological agent. While the inventory of cases is clearly incomplete, it provides an empirical basis for addressing a number of important questions relating to both biocrimes and bioterrorism. This material should enable policymakers concerned with bioterrorism to make more informed decisions. In the course of this project, the author has researched over 270 alleged cases involving biological agents. This includes all incidents found in open sources that allegedly occurred during the 20th Century. While the list is certainly not complete, it provides the most comprehensive existing unclassified coverage of instances of illicit use of biological agents.