The European Spallation Source (ESS) will be the world’s most powerful next generation neutron source. It consists of a linear accelerator, target, and instruments for neutron experiments. The linac is designed to accelerate protons to a final energy of 2.5 GeV, with a design beam power of 5 MW, for collision with a target used to pro- duce a high neutron flux. Several stages of RF acceleration are required, each using a different technology. The high beam current and power require a high degree of control of the accelerating RF, and the specification that no more than 1 W/m of losses will be experienced means that the excita- tion and decay of the higher order modes must be very well understood. Experiences at other high power machines also imply that an understanding of the generation and subse- quent trajectories of any field-emitted electrons should be understood. Thermal detuning of the HOM couplers due to multipacting is a serious concern here. This paper will outline the RF modeling plans - including the construction of mathematical models, simulations of HOMs, and mul- tipacting - during the current Accelerator Design Update phase, and will discuss several important issues for ESS.