Terminological systems, including coding and classification systems, are used in electronic medical record systems to facilitate the interpretation of structured data by providing terms and codes with a relatively precise meaning. When a clinician selects a term or code from such system and enters it in the medical record of a patient, then, from an ontological perspective and as a consequence of how terminological systems are currently integrated in electronic medical record systems, an assertion has been made to the effect that the patient exhibits, or exhibited, some phenomenon of type T. It is however left unspecified which phenomenon in particular is of the designated type T. In other words: such records contain explicit references, i.e. the terms or codes, but the referents of these references are not explicitly identified! Because referents can be referenced in many different ways, types used as references can be about many referents, and referents may change so they become of a different type, data analytics application which rely on types only are prone to drawing erroneous conclusions. Referent Tracking is a methodology for data management which allows assertions only to be made with explicit reference to the referents they are about. This chapter offers an introduction to the principles upon which the methodology rests and how these principles can be applied to improve the quality of the problem list in medical records.