The European Spallation Source (ESS) is an intergovernmental project building a multidisciplinary research laboratory based upon the world’s most powerful neutron source. The facility will be built in Lund, Sweden. The ESS will use a linear accelerator which will deliver protons with 5 MW of power to the target at 2.5 GeV, with a nominal current of 50 mA. The superconducting part of the linac consists of a total of 208 niobium cavities cooled with superfluid helium at 2 K, with 120 of these cavities in the high beta section. They are grouped in cryomodules that hold 4 cavities each and are supplied separately from the cryogenic transfer line. This presentation will show the basic design of the cryomodules and list the considerations that have lead to the decision to use exactly four cavities per cryomodule.