The phosphagens are energy storage compounds, also known as high energy phosphate compounds, are chiefly found in muscular tissue in animals. They allow a high energy phosphate pool to be maintained in a concentration range which, if it all were ATP, would create problems due to the ATP consuming reactions in these tissues. Muscle tissues will have sudden demands for lots of energy. These compounds can maintain a reserve of high energy phosphates that can kick in as needed, to provide this energy that could not be immediately supplied by glycolysis or oxidative phosphorylation.

The actual biomolecule used as a phosphagen is dependent on the organism. In mammals, the specific compound is creatine. Creatine phosphate, or phosphocreatine, is made from ATP by the enzyme creatine kinase in a reversible reaction:

• Creatine + ATP <--> creatine phosphate + ADP (this reaction is Mg⁺⁺ dependent)

Other compounds used are arginine, taurocyamine , or lombricine .