The issue of responsibility for computer failures in critical systems is addressed, taking medical information systems as an example. Three basic modes of ethical analysis are defined and used to analyze the ethical questions raised for such systems. The first, called normative ethics, seeks to develop and justify rules for right conduct. The second, called the ethics of virtue, asks what kind of person does the right thing; thus, it concerns questions of character. The third mode, called social ethics, recognizes that values and choices are not only expressed in individual actions, but are embodied effectively in social structures, and asks what structures are needed to support values such as justice and respect for human life and dignity.