This volume contains the papers presented at the Second International Work­ ing Conference on Dependable Computing for Critical Applications, sponsored by IFIP Working Group lOA and held in Tucson, Arizona on February 18-20, 1991. In keeping with the first such conference on this topic, which took place at the University of California, Santa Barbara in 1989, this meeting was like­ wise concerned with an important basic question: Can we rely on Computers? In more precise terms, it addressed various aspects of computer system de­ pendability, a broad concept defined as th'e trustworthiness of computer service such that reliance can justifiably be placed on this service. Given that this term includes attributes such as reliability, availability, safety, and security, it is our hope that these papers will contribute to further integration of these ideas in the context of critical applications. The program consisted of 20 papers and three panel sessions. The papers were selected from a total of 61 submissions at a November 1990 meeting of the Program Committee in Ann Arbor, Michigan. We were very fortunate to have a broad spectrum of interests represented, with papers in the final program coming from seven different countries, representing work at universities, corporations, and government agencies. The process was greatly facilitated by the diligent work of the Program Committee and the quality of reviews provided by outside referees. In addition to the paper presentations, there were three panel sessions or­ ganized to examine particular topics in detail.