|Publisher||Back Bay Books|
|Followed by||On Combat: The Psychology and Physiology of Deadly Conflict in War and in Peace|
On Killing: The Psychological Cost of Learning to Kill in War and Society is a book by Dave Grossman exploring the psychology of the act of killing and the military and law enforcement establishments' attempt to understand and deal with the consequences of killing.
The book is based on SLA Marshall's studies from World War II, which proposed that contrary to popular perception, the majority of soldiers in war do not ever fire their weapons and that this is due to an innate resistance to killing. Based on Marshall's studies the military instituted training measures to break down this resistance and successfully raised soldiers' firing rates to over ninety percent during the war in Vietnam.
Grossman points out that there are great psychological costs that weigh heavily on the combat soldier or police officer who kills if they are not mentally prepared for what may happen; if their actions (killing) are not supported by their commanders and/or peers; and if they are unable to justify their actions (or if no one else justifies the actions for them). Note: the second edition of this book, along with an audio version, appeared in 2009.
Robert Engen, in a paper for the Canadian Military Journal critiquing On Killing, both praised and criticized Grossman's works, saying: "On Killing and On Combat form an excellent starting point, there are too many problems with their interpretation for them to be considered the final word on the subject." Grossman's response to Engen, printed in the same journal, addresses the criticisms by showing that SLA Marshall's findings, even after having doubt cast on their methodology, have borne out in further scientific studies and real world experience, and furthermore, have been the cornerstone of military and police training for over a half century. 
- Engen, Robert. "Killing for Their Country: A New Look At 'Killology'". Canadian Military Journal 9 (2).
- Steuter, Erin; Wills, Deborah (15 July 2009). At War with Metaphor: Media, Propaganda, and Racism in the War on Terror. Lexington Books. p. 57. ISBN 978-0-7391-3031-5.
- Freeman, Sharon Morgillo; Moore, Bret A; Freeman, Arthur; Edd Arthur Freeman, Edd, Abpp (3 June 2009). Living and Surviving in Harm's Way: A Psychological Treatment Handbook for Pre- and Post-Deployment of Military Personnel. Taylor & Francis. p. 21. ISBN 978-1-135-85934-3.