Liquid cooling systems and apparatus are presented. A number of embodiments are presented. In each embodiment a heat transfer system capable of engaging a processor and adapted to transfer heat from the processor is implemented. A variety of embodiments of the heat transfer system are presented. For example, several embodiments of a direct-exposure heat transfer system are presented. In addition, several embodiments of a multi-processor heat transfer systems are presented. Lastly, several embodiments of heat transfer systems deployed in circuit boards are shown. Each of the heat transfer systems is in liquid communication with a heat exchange system that receives heated liquid from the heat transfer system and returns cooled liquid to the heat transfer system.