Heat sink and heat sink arrangements are provided for an electronic device immersed in a liquid coolant. A heat sink may comprise: a base for mounting on top of a heat-transmitting surface of the electronic device and transferring heat from the heat-transmitting surface; and a retaining wall extending from the base and defining a volume. A heat sink may have a wall arrangement to define a volume, in which the electronic device is mounted. A heat sink may be for an electronic device to be mounted on a surface in a container, in an orientation that is substantially perpendicular to a floor of the container. Heat is transferred from the electronic device to liquid coolant held in the heat sink volume. A cooling module comprising a heat sink is also provided. A nozzle arrangement may direct liquid coolant to a base of the heat sink.