The Traveller Companion is a compilation of variant and alternate rules. Variant Traveller creation systems allow specific tailoring, whilst alternate combat rules introduce hit locations and wound severity. Rules for hunger, thirst, temperature, atmospheric and water pressure effects make the universe more realistic, not to mention more hazardous. Encounters with animals and vehicles are also covered, along with the consequences of lawbreaking on personal and starship-operations scales.

The game universe is expanded with advice on using tools such as Travellermap and the Traveller Wiki, interpreting UWP data to flesh out a world, and additional spaceport types. The Companion also contains rules for starship operations in gas giant atmospheres, travelling slower than light and in jump space, and making space travel more hazardous with minefields, missile salvoes, and new starship weapons.

Also included are rules for entirely different styles of play such as narrative task resolution and the mundane events resolution system. The Traveller Companion is a complete toolkit allowing referees to pick and choose enhancements to their own campaigns, giving greater depth and weight to their adventures.