Some examples of expandable floating city are introduced, consolidating box-type modular floating structures (MFS) with square or equilateral triangle plane shape. Since reduction of wave-induced motions of the floating city is important for safety and comfort to use it for residential purpose, an overview is written of existing methods available for accurate computation of hydrodynamic forces and resulting motions. Mathematical modeling is also described for mechanical interaction forces originating from the mooring, connector, and wave energy converter equipped with power take-off system. Effects of elasticity in the MFS system are briefly reviewed.