Metallic glass is a promising metallic material with many unique properties, and also considered as a model system to study the mysteries of amorphous materials. Recently, many experimental and simulation results supported the existence of “flow unit” in metallic glass. In this paper, we review the background, the theoretical and experimental evidences of flow unit model. Flow units are considered as those loosely packed regions embedded inside the elastic matrix and behave like viscous liquid. Compared with the matrix, flow unit regions have low modulus and strength, low viscosity, high atomic mobility and stand in the saddle points on energy landscape. Therefore, flow units can be treated as dynamical defects in metallic glass. The feature, activation and evolution process of flow unit region in metallic glass as well as their correlation with property in metallic glass are also reviewed. Through dynamical mechaincal methods like dynamical mechanical spectra and stress relaxation, flow unit region and its properties can be distinguished and studied. A three-parameter physical model is proposed to describe the mechnical behaivors of flow units. The activations and evolutions of flow unit under different temperature and strain conditions are studied. A three-stage evolution process is found and the relation with mechanical performance and relaxation behavior is established. The characteristics of flow units are also related to various properties of metallic glass, like plasticity, strength, fracture and boson peaks. By using the thermal, mechanical and high pressure aging procedues, the properties of metallic glass can be manipulated as desired through adjusting the density of flow units. We show that the flow unit model not only helps to understand the mechanism behind many long-standing issues like deformation, glass transition dynamic relaxations, and the connection between strucutre and properties and performance of metallic glasses, but also is crucial for tuning and designing the properties of metallic glasses.