The Spinward Marches lie on the edge of Imperial space, and area that borders rival interstellar governments that directly oppose any further expansion. Far from the major centres of power the local rulers have more practical power than the Emperor himself. It is here that bands of daring individuals can take advantage of the hostile factions in order to carve out a fortune for themselves. The Spinward Marches offer untold asventure for those willing to seek it out. The book details the history of the sector and how the various governments interact. The role of mega-corporations is also discussed with their rivalries causing almost as much conflict as the clash of the mighty empires. The various alien races that can be found in the sector are also looked at as is how they interact with the main powers. Many planets are given a short description with many plot links that Games Masters can use for their own campaigns. A number of planets are given more detailed descriptions, with some of the more notable being so described. This sourcebook for the Traveller game serves a springboard for any new campaign set during the Third Imperium.