The nonlinear response characteristics of a tethered/moored spar platform in regular and irregular waves are investigated. A series of experiments with a 1:55 scale model was conducted in the OTRC three-dimensional deepwater model basin to observe the motion characteristics of this kind of platform in both survival and operational conditons with and without currents and winds. The spar model presently used is relatively small compared to the large spar studied in Ran et al. (1995). The small spar was supported by a vertical tether and 6 spread mooring lines. In parallel with this experimental program, a time-domain coupled nonlinear motion analysis computer program was developed to solve both the static and dynamic behaviors of a moored compliant platform as an integrated system. In particular, an efficient global-coordinate-based dynamic finite element program was developed to simulate the nonlinear tether/mooring responses. This program was successfully coupled with previously developed time-domain hull motion programs by imposing adequate boundary conditions at the attachment points. Using this program, coupled dynamic analysis results are obtained, and they are compared with uncoupled analysis results to see the effects of tethers and mooring lines on hull motions and vice versa. The numerical results are also compared with the large-scale experiments.