Hormesis is a dose-response phenomenon which is characteristic of a low dose stimulation. Large amounts of data indicate that hormesis starts to occur immediately below the no observed adverse effect levels (NOAEL) and the maximum stimulatory response is generally approximately 30%-60% greater than the control, which exists objectively. This article provides an overview of hormesis, mainly including its concept, mechanisms, quantitative features and the fitting models. Also the examples of the model fitting are given. Finally, the key issues and orientation of research work on hormesis are also illustrated and proposed.