This book provides the relationship between the static stiffness and modal stiffness of structures. The applications of the use of these relationships will be also presented as reveled in physical structures. In the understanding of a structure, the stiffness of the structure is one measure of the ability of this structure, so that an understanding of the stiffness is very important. ‘Stiffness'' is the ability of a structure to receive the loads are compared with the deformation that occurs in these structures. The higher the value the greater rigidity of the structure the ability to receive loads. The static and dynamic loads are two type of loads which the first one is relatively simple than the second one. Exploring the relationship between the static stiffness and modal stiffness of a structure is the first main content of this book. The static stiffness and modal stiffness describes how stiff a structure is from different aspects. Establishing the relationship between the two stiffness will allow some results or methods used in structural dynamic to be used for solving static problems, and vice verse.