Existent in all living organisms, enzymes often appear in the blood. Some enzymes, such as those involved in blood clotting, are normally present in large amounts. Others appear in the blood only as a result of tissue and/or cellular damage. Therefore, measuring the activity of serum enzymes may be useful in confirming the diagnosis of a particular disease or a physiological abnormality. Given their importance for clinical practice, the authors have proposed that besides aspects related to the structure, nomenclature, general properties, kinetics and mechanism of action, the main aspects regarding the clinical importance of enzymes should also addressed.