Dynamic Stiffness and Substructures models a complex dynamic system and offers a solution to the advanced dynamical problem associated with the effects of wind and earthquakes on structures. Since the system matrices are inevitably frequency dependant, those are exclusively considered in this publication. The relation between the frequency matrices by the Leung's theorem is most important in the development of efficient algorithms for the natural modes. This new approach was developed by the author over the past 15 years. It offers practising engineers and researchers a wide choice for structural modelling and analysis. Abundant numerical examples enable the reader to understand the theorem and to apply the methods.