Perfect Soldiers: The Hijackers: Who They Were, Why They Did It (also subtitled The 9/11 Hijackers: Who They Were, Why They Did It) is a 2005 book by journalist Terry McDermott. The book discusses the 9/11 hijackers, focusing on their backgrounds and what led them to commit the September 11 attacks. McDermott was an investigative reporter for the Los Angeles Times. Much of the book's information is taken from the hijackers' acquaintances, interviewed by McDermott, the 9/11 Commission Report, and other documentation. The book took three years for McDermott to write and research.

The book is split into three sections: the first covering the hijackers themselves, the second focusing on the political environment and background that shaped them, and the third focusing on the plot itself. Several appendixes include related documents, including Osama bin Laden's 1996 declaration of war against the US and the instructions left behind for the hijackers. Perfect Soldiers received a largely positive reception, with reviewers praising its research, writing, and insight into the hijackers. Several reviews called it a standout among the many books about 9/11.

Terry McDermott was a national correspondent and investigative reporter for the Los Angeles Times. He was formerly a writing coach and reporter for several other newspapers. The book took him three years to write and research. In researching for the book, McDermott interviewed several of the hijacker's acquaintances, from which much of the book's information is sourced from. Other information is taken from the 9/11 Commission Report, or FBI records and prior testimony.

Perfect Soldiers was first published by HarperCollins in 2005. Its first edition had 330 pages. Some prints were subtitled The Hijackers: Who They Were, Why They Did It, while other editions carried the subtitle The 9/11 Hijackers: Who They Were, Why They Did It.

The book's title was taken from a quote from mystery writer Dashiell Hammett's novel The Dain Curse, included after the title page: "He was the perfect soldier: he went where you sent him stayed where you put him, and had no idea of his own to keep him from doing exactly what you told him."

In a preface, McDermott argues that the perpetrators of the September 11 attacks were "regrettably, I think, fairly ordinary men. I say this is regrettable because it was their ordinariness that makes it much more likely there are a great many more men just like them." He also says that while he predicts the book will upset people in its effort to understand the motivation of the terrorists, "a primary task — and great joy, too — of the journalist is to empathize, to try to understand the way the world appeared to the people being written about."

The book explains the rise of militant Islam and gives a background on al-Qaeda and its rise. Following a prologue, Perfect Soldiers is split into three parts: "Soldiers", "The Engineer", and "The Plot". "Soldiers" profiles the hijackers and their histories. Most of the book focuses on the hijackers, their time in Europe, and their radicalization at Hamburg's fundamentalist Al-Quds Mosque. Particular focus is given to three of the hijackers: Mohamed Atta, Ziad Jarrah, and Marwan al-Shehhi.

The second part covers the Soviet–Afghan War, the Taliban's rising success, and the background to the movements in the Muslim world that gave rise to al-Qaeda and Osama bin Laden. The third and final section covers the 9/11 plot itself. McDermott concludes that the attack was successful in part due to al-Qaeda's small size, and says the terrorists made numerous mistakes in planning the attack: "[Al-Qaeda] made mistakes all the time. It didn't get caught because the government with which it was dealing made more of them." The final chapter, "That Day", covers the timeline and events of the attack itself.