The notion that the matter formed in heavy-ion collisions at RHIC has extremely low viscosity was based on the success of ideal fluid hydrodynamical models. In this talk I will review ideal fluid models used to describe heavy-ion collisions, and their uncertainties to show how large dissipative corrections to ideal model results the data actually allows. In particular I will discuss the effects of equation of state and freeze-out process. I will also briefly discuss the recent calculations applying viscous hydrodynamics to heavy-ion collisions.
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