Nonlinear Interactions provides a coherent and unified treatment of analytical, computational, and experimental methods and concepts of modal interactions. This book is an obvious extension of Ali Nayfeh's well-known book Applied Nonlinear Dynamics (with Bala Balachandran). These methods are used to explore and unfold in a unified manner the fascinating complexities in nonlinear dynamical systems. The systems discussed are drawn from fluid mechanics and structural dynamics. Nonlinear interactions between high-frequency and low-frequency modes are of great practical importance. Through the mechanisms discussed in this book, energy from high-frequency sources can be transferred to the low-frequency modes of supporting structures and foundations, and the result can be harmful large-amplitude oscillations that decrease their fatigue lives. On the other hand, these mechanisms can be exploited to transfer the energy from a system to a sacrificial subsystem and hence decrease considerably the vibrations of the main system and increase its fatigue life.