Mooring systems employed for floating offshore structures commonly produce nonlinear load- excursion characteristics. The nonlinear compliance and the consequent amplitude-dependent natural frequency, influence the vortex-induced motion of the structure. Linear stiffness modelling of catenary moored structure response has been shown to produce significant uncertainties regarding the onset and severity of flow-induced vibrations. Little guidance regarding this problem is available in the academic literature or offshore industry design codes. This book addresses this current dearth by presenting the results of several fundamental experimental investigations of the effect of nonlinear compliance on vortex-induced motions. The conclusions of these investigations have potentially significant bearing on the way in which highly nonlinear compliant systems are treated. This book will be particularly useful for researchers and students working in the field of flow-structure interaction and more generally to offshore engineers dealing with systems that cannot be adequately modelled using linear approximations.