Tomorrow, When the War Began is the first book in the Tomorrow series by John Marsden. It was published in 1993, and is a young adult invasion novel, detailing a high-intensity invasion and occupation of Australia by a foreign power. The novel is told in first person perspective by the main character, a teenage girl named Ellie Linton, who is part of a small band of teenagers waging a guerrilla war on the enemy garrison in their fictional home town of Wirrawee.

The Tomorrow series is a series of seven young adult invasion novels written by Australian writer John Marsden, detailing the invasion and occupation of Australia by a foreign power. The novels are related from the first-person perspective by Ellie Linton, a teenage girl, who is part of a small band of teenagers waging a guerrilla war on the enemy soldiers in the region around their fictional home town of Wirrawee. The name of the series is derived from the title of the first book, Tomorrow, When the War Began.