This volume in the CHC series is an overview of the critical information structures and standards required to build multifunctional electronic medical records (EMR). Description and critique of present clinical coding systems is emphasized, followed by the development of ideal design criteria required for a practical classification environment to support the electronic capture, presentation, and analyses of patient observations, findings, and events. The relationship of medical knowledge representation to patient data classification is examined, in the context of integrating clinical decision support systems and contextually appropriate guidelines into EMR systems. The role of messaging and content standards for the EMR is considered, covering the existing standards today, and those under active development that are likely to influence system implementation and functionality in the near future.