A new kind of passive mechanical damper, tuned liquid damper (TLD), is studied that relies upon the motion of shallow liquid in a rigid tank for changingthe dynamic characteristics of a structure and dissipating its vibration energy. Anonlinear model of two-dimensional liquid motion inside a rectangular TLD sub-jected to horizontal motion is developed on the basis of shallow-water wave theory,where the damping of the liquid motion is included semianalytically. Using themodel, the response of a structure with TLD is also computed. The liquid motioninside the TLD under harmonic base excitation and, furthermore, the response ofa single-degree-of-freedom structure with TLD, subjected to harmonic externalforce, are experimentally investigated. The agreement is good between the exper-iment and the prediction.