Gettysburg, fought over three days at the beginning of July 1863, is the subject of the third of Mark Adkin's magisterial 'Battle Companions' which have been universally acclaimed as comprehensive and definitive works, distinguished by their superb illustrations and selling in impressive quantities. The Waterloo Companion (2001, ISBN 978 1 85410 764 0) has so far sold 23,000 copies while The Trafalgar Companion (2005, ISBN 978 1 84513 018 3) has sold 16,500 to date. After a prologue describing the Union defeat at Chancellorsville and the death of the Confederates' greatest general, 'Stonewall' Jackson, the book begins with a chapter on the orders of battle showing, in diagrammatic form, unit strengths, commanders down to regimental and battery level, and casualties for all the units engaged. There then follow chapters on each of the arms - infantry, cavalry, artillery and other arms such as signals, medical and transport - followed by a chapter on the military and political command and control on both sides. The next chapter covers the campaign leading up to the battle followed by one on the battlefield itself. A single chapter is then devoted to each of the three days fighting, starting with a summary of the situation the rival generals faced at the start of the day and their plans and then describing in detail the phases of the day's fighting and its decisive 'highlights' such as the defence of Little Round Top on the second day and Pickett's Charge on the third, concluding with a section on the historical controversies' that have subsequently emerged. The book is completed by a chapter on the battle's aftermath, an epilogue on the 50th anniversary celebrations in 1913 and appendices. Throughout, the main text is supplemented by 'boxes' featuring curious or little-known facts or the first-hand accounts of participants.