As its name implies this book deals with clinical information systems. The clinical information system (or CIS) is an automated system with a long term database containing clinical information used for patient care. This definition excludes business systems (no clinical data) physiological monitoring systems (no long term database) and many research systems (not used in patient care). The theses of this book are (a) that CIS technology is mature (b) that the CIS will have a major impact upon patient care and the health delivery system and (c) that the number of commercial systems which now offer these potential benefits is very small. The objective of this book is to establish the above theses and thereby (a) inform both users and developers (b) increase the demand for more sophisticated products and finally (c) provide marketplace incentives to advance the state of the art. The CIS is an application of computer technology for a specific class of problems. Its development requires a knowledge of the technology with an understanding of the application area. As with any tool-based application the scope of the product will be limited by the capability of the tool. In the case of the CIS reliable computers with comprehensive database facilities became com- mercially available in the early 1970s. By the mid 1970s there was a maturation of the literature and evaluations of 5-years use began to appear. As will be shown there have been surprisingly few new ideas introduced since the 1970s.