Collective Dynamics at RHIC

The property of the ``perfect liquid'' created at RHIC is probed with anisotropic flow measurements. Different initial conditions and their consequences on flow measurements are discussed. The collectivity is shown to be achieved fast and early. The thermalization is investigated with the ratio of $v_4/v_2^2$. Measurements from three sectors of soft physics (HBT, flow and strangeness) are shown to have a simple, linear, length scaling. Directed flow is found to be independent of system size.