From earliest times, the ideas of ill health and its treatment have been wedded to those of the observation and interpretation of data. Whether we consider the disease descriptions and guidelines for management in early Greek literature or the modern physician’s use of complex laboratory and X-ray studies, it is clear that gathering data and interpreting their meaning are central to the health care process. A textbook on computers in biomedicine will accordingly refer time and again to issues in data collection, storage, and use. This chapter lays the foundation for this recurring set of issues that is pertinent to all aspects of the use of computers in biomedicine, both in the clinical world and in applications related to biology and human genetics. If data are central to all medical care, it is because they are crucial to the process of decision making (as described in detail in Chapters 3 and 4). In fact, simple reflection will reveal that all medical care activities involve gathering, analyzing, or using data. Data provide the basis for categorizing the problems a patient may be having or for identifying subgroups within a population of patients. They also help a physician to decide what additional information is needed and what actions should be taken to gain a greater understanding of a patient’s problem or to treat most effectively the problem that has been diagnosed.