The introduction of information systems in health progressively led tï coding systems. The purposes of these systems are: recording causes of death, coding diseases and procedures, etc. The most important medical coding system in our days is ICD (International Classification of Diseases). Other coding systems that health professionals use are: SNOMED, LOINC, MeSH, UMLS, DSM, DRG and HCPCS. There are also many Nursing Classification Systems, such as: NANDA, NIC, NOC, ICNP, Omaha and HHCC. This chapter describes these coding systems and their advantages.