Particulate systems are prone to electrostatic charging, which has important implications in a number of technological areas and natural occurrences. The charging of particles in both of these contexts is the result of an age-old phenomenon known as triboelectric charging that describes the transfer of charged species between material surfaces due to contact or rubbing. In comparison to bulk materials, particle systems are characterized by several unique aspects that make this area of study particularly challenging, including their inherently large surface to volume ratios which make them more susceptible to charging and their low mass which can lead to their lifting when electric forces exceed gravitational forces. Perhaps, most curious is the universal observation of charge segregation for particles of the same material where there is no clear driving force for charge transfer. In this review, we present a summary of key issues and recent progress relating to triboelectric charging of particle systems, including potential mechanisms for charge transfer, how collision dynamics plays a role, and theoretical and experimental support for charge segregation.