Isekai (Japanese for 'another world') is a genre where a character from the modern world is transported, summoned, or reincarnated into a fantasy setting, usually carrying knowledge or abilities from their previous life. The genre exploded through Japanese light novels and manga, and it's now a major force in English-language web fiction. The protagonist's outsider knowledge (modern science, game mechanics, genre awareness) becomes their advantage in the new world.
Isekai has become one of the most popular frameworks in web fiction and self-publishing. Its conventions (the summoning, the status screen, the cheat skill) form a shared vocabulary that millions of readers understand. If you're writing portal fantasy or LitRPG, understanding isekai's tropes helps you meet reader expectations or subvert them effectively.
A foundational modern isekai: a man reincarnated as a child in a fantasy world, determined to live his second life better.
Isekai subverted: the protagonist's 'cheat ability' is dying and returning to a save point, making power-fantasy into horror.
A king reincarnated as a child in a new world, blending isekai with Western progression fantasy.
Write the first scene after a character arrives in a fantasy world. They have exactly one piece of knowledge from their old life that gives them an advantage (cooking, engineering, medicine, something specific). Show them using it in a situation where it shouldn't apply but does. Make the application creative and the consequences real.