We consider the problems of dynamical stability of multiple stars. The stability criteria for triple systems are reviewed. A new classification scheme for multiple stars is given: non-hierarchical systems (probably unstable); high-hierarchical systems (probably stable); low-hierarchical systems (intermediate class of objects). We discuss the methods to study dynamical stability of concrete multiple stars taking into account the uncertainties in orbital elements and masses of components. The analysis of dynamical stability for actual multiple stars has allowed to find some candidates in unstable systems.