Zettelkasten is a note taking methodology which involves taking notes in a structured way so that notes are concise, self-contained, and can be linked to form new connections. It emphasizes rewriting content you've consumed in your own words and linking it to the source and other notes to develop understanding. The words "zettel" and "kasten" roughly translate to "slip box" and is attributed to Niklas Luhman who developed the system and used it produce 70 books and hundreds of articles on a wide range of subjects.